Chapter 5. Basic O/R Mapping

5.1. Mapping declaration

Object/relational mappings are usually defined in an XML document. The mapping document is designed to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations and not table declarations.

Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.

Here is an example mapping:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
      "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
          "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

        <class name="Cat"
            table="cats"
            discriminator-value="C">

                <id name="id">
                        <generator class="native"/>
                </id>

                <discriminator column="subclass"
                     type="character"/>

                <property name="weight"/>

                <property name="birthdate"
                    type="date"
                    not-null="true"
                    update="false"/>

                <property name="color"
                    type="eg.types.ColorUserType"
                    not-null="true"
                    update="false"/>

                <property name="sex"
                    not-null="true"
                    update="false"/>

                <property name="litterId"
                    column="litterId"
                    update="false"/>

                <many-to-one name="mother"
                    column="mother_id"
                    update="false"/>

                <set name="kittens"
                    inverse="true"
                    order-by="litter_id">
                        <key column="mother_id"/>
                        <one-to-many class="Cat"/>
                </set>

                <subclass name="DomesticCat"
                    discriminator-value="D">

                        <property name="name"
                            type="string"/>

                </subclass>

        </class>

        <class name="Dog">
                <!-- mapping for Dog could go here -->
        </class>

</hibernate-mapping>

We will now discuss the content of the mapping document. We will only describe, however, the document elements and attributes that are used by Hibernate at runtime. The mapping document also contains some extra optional attributes and elements that affect the database schemas exported by the schema export tool (for example, the not-null attribute).

5.1.1. Doctype

All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above, in the directory hibernate-x.x.x/src/org/hibernate, or in hibernate3.jar. Hibernate will always look for the DTD in its classpath first. If you experience lookups of the DTD using an Internet connection, check the DTD declaration against the contents of your classpath.

5.1.1.1. EntityResolver

Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a custom org.xml.sax.EntityResolver implementation with the SAXReader it uses to read in the xml files. This custom EntityResolver recognizes two different systemId namespaces:

a hibernate namespace is recognized whenever the resolver encounters a systemId starting with http://hibernate.sourceforge.net/. The resolver attempts to resolve these entities via the classloader which loaded the Hibernate classes.
a user namespace is recognized whenever the resolver encounters a systemId using a classpath:// URL protocol. The resolver will attempt to resolve these entities via (1) the current thread context classloader and (2) the classloader which loaded the Hibernate classes.

The following is an example of utilizing user namespacing:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [
    <!ENTITY types SYSTEM "classpath://your/domain/types.xml">
]>

<hibernate-mapping package="your.domain">
    <class name="MyEntity">
        <id name="id" type="my-custom-id-type">
            ...
        </id>
    <class>
    &types;
</hibernate-mapping>

Where types.xml is a resource in the your.domain package and contains a custom typedef.

5.1.2. Hibernate-mapping

This element has several optional attributes. The schema and catalog attributes specify that tables referred to in this mapping belong to the named schema and/or catalog. If they are specified, tablenames will be qualified by the given schema and catalog names. If they are missing, tablenames will be unqualified. The default-cascade attribute specifies what cascade style should be assumed for properties and collections that do not specify a cascade attribute. By default, the auto-import attribute allows you to use unqualified class names in the query language.

<hibernate-mapping
         schema="schemaName"                          
         catalog="catalogName"                        
         default-cascade="cascade_style"              
         default-access="field|property|ClassName"    
         default-lazy="true|false"                    
         auto-import="true|false"                     
         package="package.name"                       
 />

	`schema` (optional): the name of a database schema.
	`catalog` (optional): the name of a database catalog.
	`default-cascade` (optional - defaults to `none`): a default cascade style.
	`default-access` (optional - defaults to `property`): the strategy Hibernate should use for accessing all properties. It can be a custom implementation of `PropertyAccessor`.
	`default-lazy` (optional - defaults to `true`): the default value for unspecified `lazy` attributes of class and collection mappings.
	`auto-import` (optional - defaults to `true`): specifies whether we can use unqualified class names of classes in this mapping in the query language.
	`package` (optional): specifies a package prefix to use for unqualified class names in the mapping document.

If you have two persistent classes with the same unqualified name, you should set auto-import="false". An exception will result if you attempt to assign two classes to the same "imported" name.

The hibernate-mapping element allows you to nest several persistent <class> mappings, as shown above. It is, however, good practice (and expected by some tools) to map only a single persistent class, or a single class hierarchy, in one mapping file and name it after the persistent superclass. For example, Cat.hbm.xml, Dog.hbm.xml, or if using inheritance, Animal.hbm.xml.

5.1.3. Class

You can declare a persistent class using the class element. For example:

<class
        name="ClassName"                              
        table="tableName"                             
        discriminator-value="discriminator_value"     
        mutable="true|false"                          
        schema="owner"                                
        catalog="catalog"                             
        proxy="ProxyInterface"                        
        dynamic-update="true|false"                   
        dynamic-insert="true|false"                   
        select-before-update="true|false"             
        polymorphism="implicit|explicit"              
        where="arbitrary sql where condition"         
        persister="PersisterClass"                    
        batch-size="N"                                
        optimistic-lock="none|version|dirty|all"      
        lazy="true|false"                             (16)
        entity-name="EntityName"                      (17)
        check="arbitrary sql check condition"         (18)
        rowid="rowid"                                 (19)
        subselect="SQL expression"                    (20)
        abstract="true|false"                         (21)
        node="element-name"
/>

	`name` (optional): the fully qualified Java class name of the persistent class or interface. If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.
	`table` (optional - defaults to the unqualified class name): the name of its database table.
	`discriminator-value` (optional - defaults to the class name): a value that distinguishes individual subclasses that is used for polymorphic behavior. Acceptable values include `null` and `not null`.
	`mutable` (optional - defaults to `true`): specifies that instances of the class are (not) mutable.
	`schema` (optional): overrides the schema name specified by the root `<hibernate-mapping>` element.
	`catalog` (optional): overrides the catalog name specified by the root `<hibernate-mapping>` element.
	`proxy` (optional): specifies an interface to use for lazy initializing proxies. You can specify the name of the class itself.
	`dynamic-update` (optional - defaults to `false`): specifies that `UPDATE` SQL should be generated at runtime and can contain only those columns whose values have changed.
	`dynamic-insert` (optional - defaults to `false`): specifies that `INSERT` SQL should be generated at runtime and contain only the columns whose values are not null.
	`select-before-update` (optional - defaults to `false`): specifies that Hibernate should never perform an SQL `UPDATE` unless it is certain that an object is actually modified. Only when a transient object has been associated with a new session using `update()`, will Hibernate perform an extra SQL `SELECT` to determine if an `UPDATE` is actually required.
	`polymorphism` (optional - defaults to `implicit`): determines whether implicit or explicit query polymorphism is used.
	`where` (optional): specifies an arbitrary SQL `WHERE` condition to be used when retrieving objects of this class.
	`persister` (optional): specifies a custom `ClassPersister`.
	`batch-size` (optional - defaults to `1`): specifies a "batch size" for fetching instances of this class by identifier.
	`optimistic-lock` (optional - defaults to `version`): determines the optimistic locking strategy.
(16)	`lazy` (optional): lazy fetching can be disabled by setting `lazy="false"`.
(17)	`entity-name` (optional - defaults to the class name): Hibernate3 allows a class to be mapped multiple times, potentially to different tables. It also allows entity mappings that are represented by Maps or XML at the Java level. In these cases, you should provide an explicit arbitrary name for the entity. See Section 4.4, “Dynamic models” and Chapter 18, XML Mapping for more information.
(18)	`check` (optional): an SQL expression used to generate a multi-row check constraint for automatic schema generation.
(19)	`rowid` (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use the `rowid` extra column for fast updates once this option has been set to `rowid`. A ROWID is an implementation detail and represents the physical location of a stored tuple.
(20)	`subselect` (optional): maps an immutable and read-only entity to a database subselect. This is useful if you want to have a view instead of a base table. See below for more information.
(21)	`abstract` (optional): is used to mark abstract superclasses in `<union-subclass>` hierarchies.

It is acceptable for the named persistent class to be an interface. You can declare implementing classes of that interface using the <subclass> element. You can persist any static inner class. Specify the class name using the standard form i.e. e.g.Foo$Bar.

Immutable classes, mutable="false", cannot be updated or deleted by the application. This allows Hibernate to make some minor performance optimizations.

The optional proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies that implement the named interface. The persistent object will load when a method of the proxy is invoked. See "Initializing collections and proxies" below.

Implicit polymorphism means that instances of the class will be returned by a query that names any superclass or implemented interface or class, and that instances of any subclass of the class will be returned by a query that names the class itself. Explicit polymorphism means that class instances will be returned only by queries that explicitly name that class. Queries that name the class will return only instances of subclasses mapped inside this <class> declaration as a <subclass> or <joined-subclass>. For most purposes, the default polymorphism="implicit" is appropriate. Explicit polymorphism is useful when two different classes are mapped to the same table This allows a "lightweight" class that contains a subset of the table columns.

The persister attribute lets you customize the persistence strategy used for the class. You can, for example, specify your own subclass of org.hibernate.persister.EntityPersister, or you can even provide a completely new implementation of the interface org.hibernate.persister.ClassPersister that implements, for example, persistence via stored procedure calls, serialization to flat files or LDAP. See org.hibernate.test.CustomPersister for a simple example of "persistence" to a Hashtable.

The dynamic-update and dynamic-insert settings are not inherited by subclasses, so they can also be specified on the <subclass> or <joined-subclass> elements. Although these settings can increase performance in some cases, they can actually decrease performance in others.

Use of select-before-update will usually decrease performance. It is useful to prevent a database update trigger being called unnecessarily if you reattach a graph of detached instances to a Session.

If you enable dynamic-update, you will have a choice of optimistic locking strategies:

version: check the version/timestamp columns
all: check all columns
dirty: check the changed columns, allowing some concurrent updates
none: do not use optimistic locking

It is strongly recommended that you use version/timestamp columns for optimistic locking with Hibernate. This strategy optimizes performance and correctly handles modifications made to detached instances (i.e. when Session.merge() is used).

There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level, although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but you cannot create one in the database (i.e. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression:

<class name="Summary">
    <subselect>
        select item.name, max(bid.amount), count(*)
        from item
        join bid on bid.item_id = item.id
        group by item.name
    </subselect>
    <synchronize table="item"/>
    <synchronize table="bid"/>
    <id name="name"/>
    ...
</class>

Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that queries against the derived entity do not return stale data. The <subselect> is available both as an attribute and a nested mapping element.

5.1.4. id

Mapped classes must declare the primary key column of the database table. Most classes will also have a JavaBeans-style property holding the unique identifier of an instance. The <id> element defines the mapping from that property to the primary key column.

<id
        name="propertyName"                                          
        type="typename"                                              
        column="column_name"                                         
        unsaved-value="null|any|none|undefined|id_value"             
        access="field|property|ClassName">                           
        node="element-name|@attribute-name|element/@attribute|."

        <generator class="generatorClass"/>
</id>

	`name` (optional): the name of the identifier property.
	`type` (optional): a name that indicates the Hibernate type.
	`column` (optional - defaults to the property name): the name of the primary key column.
	`unsaved-value` (optional - defaults to a "sensible" value): an identifier property value that indicates an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.
	`access` (optional - defaults to `property`): the strategy Hibernate should use for accessing the property value.

If the name attribute is missing, it is assumed that the class has no identifier property.

The unsaved-value attribute is almost never needed in Hibernate3.

There is an alternative <composite-id> declaration that allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.

5.1.4.1. Generator

The optional <generator> child element names a Java class used to generate unique identifiers for instances of the persistent class. If any parameters are required to configure or initialize the generator instance, they are passed using the <param> element.

<id name="id" type="long" column="cat_id">
        <generator class="org.hibernate.id.TableHiLoGenerator">
                <param name="table">uid_table</param>
                <param name="column">next_hi_value_column</param>
        </generator>
</id>

All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple interface. Some applications can choose to provide their own specialized implementations, however, Hibernate provides a range of built-in implementations. The shortcut names for the built-in generators are as follows:

increment: generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster.
identity: supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.
sequence: uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type long, short or int
hilo: uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.
seqhilo: uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.
uuid: uses a 128-bit UUID algorithm to generate identifiers of type string that are unique within a network (the IP address is used). The UUID is encoded as a string of 32 hexadecimal digits in length.
guid: uses a database-generated GUID string on MS SQL Server and MySQL.
native: selects identity, sequence or hilo depending upon the capabilities of the underlying database.
assigned: lets the application assign an identifier to the object before save() is called. This is the default strategy if no <generator> element is specified.
select: retrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving the primary key value.
foreign: uses the identifier of another associated object. It is usually used in conjunction with a <one-to-one> primary key association.
sequence-identity: a specialized sequence generation strategy that utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on these insert statements are disabled due to a bug in the Oracle drivers.

5.1.4.2. Hi/lo algorithm

The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm. The first implementation requires a "special" database table to hold the next available "hi" value. Where supported, the second uses an Oracle-style sequence.

<id name="id" type="long" column="cat_id">
        <generator class="hilo">
                <param name="table">hi_value</param>
                <param name="column">next_value</param>
                <param name="max_lo">100</param>
        </generator>
</id>

<id name="id" type="long" column="cat_id">
        <generator class="seqhilo">
                <param name="sequence">hi_value</param>
                <param name="max_lo">100</param>
        </generator>
</id>

Unfortunately, you cannot use hilo when supplying your own Connection to Hibernate. When Hibernate uses an application server datasource to obtain connections enlisted with JTA, you must configure the hibernate.transaction.manager_lookup_class.

5.1.4.3. UUID algorithm

The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so this is the best option without using JNI.

5.1.4.4. Identity columns and sequences

For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can use identity key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can use sequence style key generation. Both of these strategies require two SQL queries to insert a new object. For example:

<id name="id" type="long" column="person_id">
        <generator class="sequence">
                <param name="sequence">person_id_sequence</param>
        </generator>
</id>

<id name="id" type="long" column="person_id" unsaved-value="0">
        <generator class="identity"/>
</id>

For cross-platform development, the native strategy will, depending on the capabilities of the underlying database, choose from the identity, sequence and hilo strategies.

5.1.4.5. Assigned identifiers

If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can use the assigned generator. This special generator uses the identifier value already assigned to the object's identifier property. The generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do not specify a <generator> element.

The assigned generator makes Hibernate use unsaved-value="undefined". This forces Hibernate to go to the database to determine if an instance is transient or detached, unless there is a version or timestamp property, or you define Interceptor.isUnsaved().

5.1.4.6. Primary keys assigned by triggers

Hibernate does not generate DDL with triggers. It is for legacy schemas only.

<id name="id" type="long" column="person_id">
        <generator class="select">
                <param name="key">socialSecurityNumber</param>
        </generator>
</id>

In the above example, there is a unique valued property named socialSecurityNumber. It is defined by the class, as a natural key and a surrogate key named person_id, whose value is generated by a trigger.

5.1.5. Enhanced identifier generators

Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifier generation. The first aspect is database portability; the second is optimization Optimization means that you do not have to query the database for every request for a new identifier value. These two new generators are intended to take the place of some of the named generators described above, starting in 3.3.x. However, they are included in the current releases and can be referenced by FQN.

The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator which is intended, firstly, as a replacement for the sequence generator and, secondly, as a better portability generator than native. This is because native generally chooses between identity and sequence which have largely different semantics that can cause subtle issues in applications eyeing portability. org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in a different manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on the capabilities of the dialect being used. The difference between this and native is that table-based and sequence-based storage have the same exact semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-based generators. This generator has a number of configuration parameters:

sequence_name (optional, defaults to hibernate_sequence): the name of the sequence or table to be used.
initial_value (optional, defaults to 1): the initial value to be retrieved from the sequence/table. In sequence creation terms, this is analogous to the clause typically named "STARTS WITH".
increment_size (optional - defaults to 1): the value by which subsequent calls to the sequence/table should differ. In sequence creation terms, this is analogous to the clause typically named "INCREMENT BY".
force_table_use (optional - defaults to false): should we force the use of a table as the backing structure even though the dialect might support sequence?
value_column (optional - defaults to next_val): only relevant for table structures, it is the name of the column on the table which is used to hold the value.
optimizer (optional - defaults to none): See Section 5.1.6, “Identifier generator optimization”

The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which is intended, firstly, as a replacement for the table generator, even though it actually functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator, and secondly, as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the notion of pluggable optimizers. Essentially this generator defines a table capable of holding a number of different increment values simultaneously by using multiple distinctly keyed rows. This generator has a number of configuration parameters:

table_name (optional - defaults to hibernate_sequences): the name of the table to be used.
value_column_name (optional - defaults to next_val): the name of the column on the table that is used to hold the value.
segment_column_name (optional - defaults to sequence_name): the name of the column on the table that is used to hold the "segment key". This is the value which identifies which increment value to use.
segment_value (optional - defaults to default): The "segment key" value for the segment from which we want to pull increment values for this generator.
segment_value_length (optional - defaults to 255): Used for schema generation; the column size to create this segment key column.
initial_value (optional - defaults to 1): The initial value to be retrieved from the table.
increment_size (optional - defaults to 1): The value by which subsequent calls to the table should differ.
optimizer (optional - defaults to ): See Section 5.1.6, “Identifier generator optimization”

`5.1.6. Identifier generator optimization`

For identifier generators that store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value. Instead, you can group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators (Section 5.1.5, “Enhanced identifier generators” support this operation.

none (generally this is the default if no optimizer was specified): this will not perform any optimizations and hit the database for each and every request.
hilo: applies a hi/lo algorithm around the database retrieved values. The values from the database for this optimizer are expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "group number". The increment_size is multiplied by that value in memory to define a group "hi value".
pooled: as with the case of hilo, this optimizer attempts to minimize the number of hits to the database. Here, however, we simply store the starting value for the "next group" into the database structure rather than a sequential value in combination with an in-memory grouping algorithm. Here, increment_size refers to the values coming from the database.

`5.1.7. composite-id`

<composite-id
        name="propertyName"
        class="ClassName"
        mapped="true|false"
        access="field|property|ClassName">
        node="element-name|."

        <key-property name="propertyName" type="typename" column="column_name"/>
        <key-many-to-one name="propertyName class="ClassName" column="column_name"/>
        ......
</composite-id>

A table with a composite key can be mapped with multiple properties of the class as identifier properties. The <composite-id> element accepts <key-property> property mappings and <key-many-to-one> mappings as child elements.

<composite-id>
        <key-property name="medicareNumber"/>
        <key-property name="dependent"/>
</composite-id>

The persistent class must override equals() and hashCode() to implement composite identifier equality. It must also implement Serializable.

Unfortunately, this approach means that a persistent object is its own identifier. There is no convenient "handle" other than the object itself. You must instantiate an instance of the persistent class itself and populate its identifier properties before you can load() the persistent state associated with a composite key. We call this approach an embedded composite identifier, and discourage it for serious applications.

A second approach is what we call a mapped composite identifier, where the identifier properties named inside the <composite-id> element are duplicated on both the persistent class and a separate identifier class.

<composite-id class="MedicareId" mapped="true">
        <key-property name="medicareNumber"/>
        <key-property name="dependent"/>
</composite-id>

In this example, both the composite identifier class, MedicareId, and the entity class itself have properties named medicareNumber and dependent. The identifier class must override equals() and hashCode() and implement Serializable. The main disadvantage of this approach is code duplication.

The following attributes are used to specify a mapped composite identifier:

mapped (optional - defaults to false): indicates that a mapped composite identifier is used, and that the contained property mappings refer to both the entity class and the composite identifier class.
class (optional - but required for a mapped composite identifier): the class used as a composite identifier.

We will describe a third, even more convenient approach, where the composite identifier is implemented as a component class in Section 8.4, “Components as composite identifiers”. The attributes described below apply only to this alternative approach:

name (optional - required for this approach): a property of component type that holds the composite identifier. Please see chapter 9 for more information.
access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
class (optional - defaults to the property type determined by reflection): the component class used as a composite identifier. Please see the next section for more information.

The third approach, an identifier component, is recommended for almost all applications.

`5.1.8. Discriminator`

The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy mapping strategy. It declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. A restricted set of types can be used: string, character, integer, byte, short, boolean, yes_no, true_false.

<discriminator
        column="discriminator_column"                      
        type="discriminator_type"                          
        force="true|false"                                 
        insert="true|false"                                
        formula="arbitrary sql expression"                 
/>

	`column` (optional - defaults to `class`): the name of the discriminator column.
	`type` (optional - defaults to `string`): a name that indicates the Hibernate type
	`force` (optional - defaults to `false`): "forces" Hibernate to specify the allowed discriminator values, even when retrieving all instances of the root class.
	`insert` (optional - defaults to `true`): set this to `false` if your discriminator column is also part of a mapped composite identifier. It tells Hibernate not to include the column in SQL `INSERTs`.
	`formula` (optional): an arbitrary SQL expression that is executed when a type has to be evaluated. It allows content-based discrimination.

Actual values of the discriminator column are specified by the discriminator-value attribute of the <class> and <subclass> elements.

The force attribute is only useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This will not usually be the case.

The formula attribute allows you to declare an arbitrary SQL expression that will be used to evaluate the type of a row. For example:

<discriminator
    formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
    type="integer"/>

`5.1.9. Version (optional)`

The <version> element is optional and indicates that the table contains versioned data. This is particularly useful if you plan to use long transactions. See below for more information:

<version
        column="version_column"                                      
        name="propertyName"                                          
        type="typename"                                              
        access="field|property|ClassName"                            
        unsaved-value="null|negative|undefined"                      
        generated="never|always"                                     
        insert="true|false"                                          
        node="element-name|@attribute-name|element/@attribute|."
/>

	`column` (optional - defaults to the property name): the name of the column holding the version number.
	`name`: the name of a property of the persistent class.
	`type` (optional - defaults to `integer`): the type of the version number.
	`access` (optional - defaults to `property`): the strategy Hibernate uses to access the property value.
	`unsaved-value` (optional - defaults to `undefined`): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. `Undefined` specifies that the identifier property value should be used.
	`generated` (optional - defaults to `never`): specifies that this version property value is generated by the database. See the discussion of generated properties for more information.
	`insert` (optional - defaults to `true`): specifies whether the version column should be included in SQL insert statements. It can be set to `false` if the database column is defined with a default value of `0`.

Version numbers can be of Hibernate type long, integer, short, timestamp or calendar.

A version or timestamp property should never be null for a detached instance. Hibernate will detect any instance with a null version or timestamp as transient, irrespective of what other unsaved-value strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate. It is especially useful for people using assigned identifiers or composite keys.

`5.1.10. Timestamp (optional)`

The optional <timestamp> element indicates that the table contains timestamped data. This provides an alternative to versioning. Timestamps are a less safe implementation of optimistic locking. However, sometimes the application might use the timestamps in other ways.

<timestamp
        column="timestamp_column"                                    
        name="propertyName"                                          
        access="field|property|ClassName"                            
        unsaved-value="null|undefined"                               
        source="vm|db"                                               
        generated="never|always"                                     
        node="element-name|@attribute-name|element/@attribute|."
/>

	`column` (optional - defaults to the property name): the name of a column holding the timestamp.
	`name`: the name of a JavaBeans style property of Java type `Date` or `Timestamp` of the persistent class.
	`access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
	`unsaved-value` (optional - defaults to `null`): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. `Undefined` specifies that the identifier property value should be used.
	`source` (optional - defaults to `vm`): Where should Hibernate retrieve the timestamp value from? From the database, or from the current JVM? Database-based timestamps incur an overhead because Hibernate must hit the database in order to determine the "next value". It is safer to use in clustered environments. Not all `Dialects` are known to support the retrieval of the database's current timestamp. Others may also be unsafe for usage in locking due to lack of precision (Oracle 8, for example).
	`generated` (optional - defaults to `never`): specifies that this timestamp property value is actually generated by the database. See the discussion of generated properties for more information.

`Note`

<Timestamp> is equivalent to <version type="timestamp">. And <timestamp source="db"> is equivalent to <version type="dbtimestamp">

`5.1.11. Property`

The <property> element declares a persistent JavaBean style property of the class.

<property
        name="propertyName"                                          
        column="column_name"                                         
        type="typename"                                              
        update="true|false"                                          
        insert="true|false"                                          
        formula="arbitrary SQL expression"                           
        access="field|property|ClassName"                            
        lazy="true|false"                                            
        unique="true|false"                                          
        not-null="true|false"                                        
        optimistic-lock="true|false"                                 
        generated="never|insert|always"                              
        node="element-name|@attribute-name|element/@attribute|."
        index="index_name"
        unique_key="unique_key_id"
        length="L"
        precision="P"
        scale="S"
/>

	`name`: the name of the property, with an initial lowercase letter.
	`column` (optional - defaults to the property name): the name of the mapped database table column. This can also be specified by nested `<column>` element(s).
	`type` (optional): a name that indicates the Hibernate type.
	`update, insert` (optional - defaults to `true`): specifies that the mapped columns should be included in SQL `UPDATE` and/or `INSERT` statements. Setting both to `false` allows a pure "derived" property whose value is initialized from some other property that maps to the same column(s), or by a trigger or other application.
	`formula` (optional): an SQL expression that defines the value for a computed property. Computed properties do not have a column mapping of their own.
	`access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
	`lazy` (optional - defaults to `false`): specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.
	`unique` (optional): enables the DDL generation of a unique constraint for the columns. Also, allow this to be the target of a `property-ref`.
	`not-null` (optional): enables the DDL generation of a nullability constraint for the columns.
	`optimistic-lock` (optional - defaults to `true`): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
	`generated` (optional - defaults to `never`): specifies that this property value is actually generated by the database. See the discussion of generated properties for more information.

typename could be:

The name of a Hibernate basic type: integer, string, character, date, timestamp, float, binary, serializable, object, blob etc.
The name of a Java class with a default basic type: int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob etc.
The name of a serializable Java class.
The class name of a custom type: com.illflow.type.MyCustomType etc.

If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4. In certain cases you will need the type attribute. For example, to distinguish between Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.

The access attribute allows you to control how Hibernate accesses the property at runtime. By default, Hibernate will call the property get/set pair. If you specify access="field", Hibernate will bypass the get/set pair and access the field directly using reflection. You can specify your own strategy for property access by naming a class that implements the interface org.hibernate.property.PropertyAccessor.

A powerful feature is derived properties. These properties are by definition read-only. The property value is computed at load time. You declare the computation as an SQL expression. This then translates to a SELECT clause subquery in the SQL query that loads an instance:

<property name="totalPrice"
    formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p
                WHERE li.productId = p.productId
                AND li.customerId = customerId
                AND li.orderNumber = orderNumber )"/>

You can reference the entity table by not declaring an alias on a particular column. This would be customerId in the given example. You can also use the nested <formula> mapping element if you do not want to use the attribute.

`5.1.12. Many-to-one`

An ordinary association to another persistent class is declared using a many-to-one element. The relational model is a many-to-one association; a foreign key in one table is referencing the primary key column(s) of the target table.

<many-to-one
        name="propertyName"                                          
        column="column_name"                                         
        class="ClassName"                                            
        cascade="cascade_style"                                      
        fetch="join|select"                                          
        update="true|false"                                          
        insert="true|false"                                          
        property-ref="propertyNameFromAssociatedClass"               
        access="field|property|ClassName"                            
        unique="true|false"                                          
        not-null="true|false"                                        
        optimistic-lock="true|false"                                 
        lazy="proxy|no-proxy|false"                                  
        not-found="ignore|exception"                                 
        entity-name="EntityName"                                     
        formula="arbitrary SQL expression"                           
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        index="index_name"
        unique_key="unique_key_id"
        foreign-key="foreign_key_name"
/>

	`name`: the name of the property.
	`column` (optional): the name of the foreign key column. This can also be specified by nested `<column>` element(s).
	`class` (optional - defaults to the property type determined by reflection): the name of the associated class.
	`cascade` (optional): specifies which operations should be cascaded from the parent object to the associated object.
	`fetch` (optional - defaults to `select`): chooses between outer-join fetching or sequential select fetching.
	`update, insert` (optional - defaults to `true`): specifies that the mapped columns should be included in SQL `UPDATE` and/or `INSERT` statements. Setting both to `false` allows a pure "derived" association whose value is initialized from another property that maps to the same column(s), or by a trigger or other application.
	`property-ref` (optional): the name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.
	`access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
	`unique` (optional): enables the DDL generation of a unique constraint for the foreign-key column. By allowing this to be the target of a `property-ref`, you can make the association multiplicity one-to-one.
	`not-null` (optional): enables the DDL generation of a nullability constraint for the foreign key columns.
	`optimistic-lock` (optional - defaults to `true`): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
	`lazy` (optional - defaults to `proxy`): by default, single point associations are proxied. `lazy="no-proxy"` specifies that the property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. `lazy="false"` specifies that the association will always be eagerly fetched.
	`not-found` (optional - defaults to `exception`): specifies how foreign keys that reference missing rows will be handled. `ignore` will treat a missing row as a null association.
	`entity-name` (optional): the entity name of the associated class.
	`formula` (optional): an SQL expression that defines the value for a computed foreign key.

Setting a value of the cascade attribute to any meaningful value other than none will propagate certain operations to the associated object. The meaningful values are divided into three categories. First, basic operations, which include: persist, merge, delete, save-update, evict, replicate, lock and refresh; second, special values: delete-orphan; and third,all comma-separated combinations of operation names: cascade="persist,merge,evict" or cascade="all,delete-orphan". See Section 10.11, “Transitive persistence” for a full explanation. Note that single valued, many-to-one and one-to-one, associations do not support orphan delete.

Here is an example of a typical many-to-one declaration:

<many-to-one name="product" class="Product" column="PRODUCT_ID"/>

The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is a complicated and confusing relational model. For example, if the Product class had a unique serial number that is not the primary key. The unique attribute controls Hibernate's DDL generation with the SchemaExport tool.

<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>

Then the mapping for OrderItem might use:

<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>

This is not encouraged, however.

If the referenced unique key comprises multiple properties of the associated entity, you should map the referenced properties inside a named <properties> element.

If the referenced unique key is the property of a component, you can specify a property path:

<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>

`5.1.13. One-to-one`

A one-to-one association to another persistent class is declared using a one-to-one element.

<one-to-one
        name="propertyName"                                          
        class="ClassName"                                            
        cascade="cascade_style"                                      
        constrained="true|false"                                     
        fetch="join|select"                                          
        property-ref="propertyNameFromAssociatedClass"               
        access="field|property|ClassName"                            
        formula="any SQL expression"                                 
        lazy="proxy|no-proxy|false"                                  
        entity-name="EntityName"                                     
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        foreign-key="foreign_key_name"
/>

	`name`: the name of the property.
	`class` (optional - defaults to the property type determined by reflection): the name of the associated class.
	`cascade` (optional): specifies which operations should be cascaded from the parent object to the associated object.
	`constrained` (optional): specifies that a foreign key constraint on the primary key of the mapped table and references the table of the associated class. This option affects the order in which `save()` and `delete()` are cascaded, and determines whether the association can be proxied. It is also used by the schema export tool.
	`fetch` (optional - defaults to `select`): chooses between outer-join fetching or sequential select fetching.
	`property-ref` (optional): the name of a property of the associated class that is joined to the primary key of this class. If not specified, the primary key of the associated class is used.
	`access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
	`formula` (optional): almost all one-to-one associations map to the primary key of the owning entity. If this is not the case, you can specify another column, columns or expression to join on using an SQL formula. See `org.hibernate.test.onetooneformula` for an example.
	`lazy` (optional - defaults to `proxy`): by default, single point associations are proxied. `lazy="no-proxy"` specifies that the property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation. `lazy="false"` specifies that the association will always be eagerly fetched. Note that if `constrained="false"`, proxying is impossible and Hibernate will eagerly fetch the association.
	`entity-name` (optional): the entity name of the associated class.

There are two varieties of one-to-one associations:

primary key associations
unique foreign key associations

Primary key associations do not need an extra table column. If two rows are related by the association, then the two table rows share the same primary key value. To relate two objects by a primary key association, ensure that they are assigned the same identifier value.

For a primary key association, add the following mappings to Employee and Person respectively:

<one-to-one name="person" class="Person"/>

<one-to-one name="employee" class="Employee" constrained="true"/>

Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You use a special Hibernate identifier generation strategy called foreign:

<class name="person" table="PERSON">
    <id name="id" column="PERSON_ID">
        <generator class="foreign">
            <param name="property">employee</param>
        </generator>
    </id>
    ...
    <one-to-one name="employee"
        class="Employee"
        constrained="true"/>
</class>

A newly saved instance of Person is assigned the same primary key value as the Employee instance referred with the employee property of that Person.

Alternatively, a foreign key with a unique constraint, from Employee to Person, can be expressed as:

<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>

This association can be made bidirectional by adding the following to the Person mapping:

<one-to-one name="employee" class="Employee" property-ref="person"/>

`5.1.14. Natural-id`

<natural-id mutable="true|false"/>
        <property ... />
        <many-to-one ... />
        ......
</natural-id>

Although we recommend the use of surrogate keys as primary keys, you should try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. It is also immutable. Map the properties of the natural key inside the <natural-id> element. Hibernate will generate the necessary unique key and nullability constraints and, as a result, your mapping will be more self-documenting.

This mapping is not intended for use with entities that have natural primary keys.

mutable (optional - defaults to false): by default, natural identifier properties are assumed to be immutable (constant).

`5.1.15. Component and dynamic-component`

The <component> element maps properties of a child object to columns of the table of a parent class. Components can, in turn, declare their own properties, components or collections. See the "Component" examples below:

<component
        name="propertyName"                 
        class="className"                   
        insert="true|false"                 
        update="true|false"                 
        access="field|property|ClassName"   
        lazy="true|false"                   
        optimistic-lock="true|false"        
        unique="true|false"                 
        node="element-name|."
>

        <property ...../>
        <many-to-one .... />
        ........
</component>

	`name`: the name of the property.
	`class` (optional - defaults to the property type determined by reflection): the name of the component (child) class.
	`insert`: do the mapped columns appear in SQL `INSERTs`?
	`update`: do the mapped columns appear in SQL `UPDATEs`?
	`access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
	`lazy` (optional - defaults to `false`): specifies that this component should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.
	`optimistic-lock` (optional - defaults to `true`): specifies that updates to this component either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when this property is dirty.
	`unique` (optional - defaults to `false`): specifies that a unique constraint exists upon all mapped columns of the component.

The child <property> tags map properties of the child class to table columns.

The <component> element allows a <parent> subelement that maps a property of the component class as a reference back to the containing entity.

The <dynamic-component> element allows a Map to be mapped as a component, where the property names refer to keys of the map. See Section 8.5, “Dynamic components” for more information.

`5.1.16. Properties`

The <properties> element allows the definition of a named, logical grouping of the properties of a class. The most important use of the construct is that it allows a combination of properties to be the target of a property-ref. It is also a convenient way to define a multi-column unique constraint. For example:

<properties
        name="logicalName"                  
        insert="true|false"                 
        update="true|false"                 
        optimistic-lock="true|false"        
        unique="true|false"                 
>

        <property ...../>
        <many-to-one .... />
        ........
</properties>

	`name`: the logical name of the grouping. It is not an actual property name.
	`insert`: do the mapped columns appear in SQL `INSERTs`?
	`update`: do the mapped columns appear in SQL `UPDATEs`?
	`optimistic-lock` (optional - defaults to `true`): specifies that updates to these properties either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when these properties are dirty.
	`unique` (optional - defaults to `false`): specifies that a unique constraint exists upon all mapped columns of the component.

For example, if we have the following <properties> mapping:

<class name="Person">
    <id name="personNumber"/>

    ...
    <properties name="name"
            unique="true" update="false">
        <property name="firstName"/>
        <property name="initial"/>
        <property name="lastName"/>
    </properties>
</class>

You might have some legacy data association that refers to this unique key of the Person table, instead of to the primary key:

<many-to-one name="person"
         class="Person" property-ref="name">
    <column name="firstName"/>
    <column name="initial"/>
    <column name="lastName"/>
</many-to-one>

The use of this outside the context of mapping legacy data is not recommended.

`5.1.17. Subclass`

Polymorphic persistence requires the declaration of each subclass of the root persistent class. For the table-per-class-hierarchy mapping strategy, the <subclass> declaration is used. For example:

<subclass
        name="ClassName"                              
        discriminator-value="discriminator_value"     
        proxy="ProxyInterface"                        
        lazy="true|false"                             
        dynamic-update="true|false"
        dynamic-insert="true|false"
        entity-name="EntityName"
        node="element-name"
        extends="SuperclassName">

        <property .... />
        .....
</subclass>

	`name`: the fully qualified class name of the subclass.
	`discriminator-value` (optional - defaults to the class name): a value that distinguishes individual subclasses.
	`proxy` (optional): specifies a class or interface used for lazy initializing proxies.
	`lazy` (optional - defaults to `true`): setting `lazy="false"` disables the use of lazy fetching.

Each subclass declares its own persistent properties and subclasses. <version> and <id> properties are assumed to be inherited from the root class. Each subclass in a hierarchy must define a unique discriminator-value. If this is not specified, the fully qualified Java class name is used.

For information about inheritance mappings see Chapter 9, Inheritance mapping.

`5.1.18. Joined-subclass`

Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass. To do this you use the <joined-subclass> element. For example:

<joined-subclass
        name="ClassName"                    
        table="tablename"                   
        proxy="ProxyInterface"              
        lazy="true|false"                   
        dynamic-update="true|false"
        dynamic-insert="true|false"
        schema="schema"
        catalog="catalog"
        extends="SuperclassName"
        persister="ClassName"
        subselect="SQL expression"
        entity-name="EntityName"
        node="element-name">

        <key .... >

        <property .... />
        .....
</joined-subclass>

	`name`: the fully qualified class name of the subclass.
	`table`: the name of the subclass table.
	`proxy` (optional): specifies a class or interface to use for lazy initializing proxies.
	`lazy` (optional, defaults to `true`): setting `lazy="false"` disables the use of lazy fetching.

A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier using the <key> element. The mapping at the start of the chapter would then be re-written as:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

        <class name="Cat" table="CATS">
                <id name="id" column="uid" type="long">
                        <generator class="hilo"/>
                </id>
                <property name="birthdate" type="date"/>
                <property name="color" not-null="true"/>
                <property name="sex" not-null="true"/>
                <property name="weight"/>
                <many-to-one name="mate"/>
                <set name="kittens">
                        <key column="MOTHER"/>
                        <one-to-many class="Cat"/>
                </set>
                <joined-subclass name="DomesticCat" table="DOMESTIC_CATS">
                    <key column="CAT"/>
                    <property name="name" type="string"/>
                </joined-subclass>
        </class>

        <class name="eg.Dog">
                <!-- mapping for Dog could go here -->
        </class>

</hibernate-mapping>

For information about inheritance mappings see Chapter 9, Inheritance mapping.

`5.1.19. Union-subclass`

A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state. In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can map each class with a separate <class> declaration. However, if you wish use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use the <union-subclass> mapping. For example:

<union-subclass
        name="ClassName"                    
        table="tablename"                   
        proxy="ProxyInterface"              
        lazy="true|false"                   
        dynamic-update="true|false"
        dynamic-insert="true|false"
        schema="schema"
        catalog="catalog"
        extends="SuperclassName"
        abstract="true|false"
        persister="ClassName"
        subselect="SQL expression"
        entity-name="EntityName"
        node="element-name">

        <property .... />
        .....
</union-subclass>

	`name`: the fully qualified class name of the subclass.
	`table`: the name of the subclass table.
	`proxy` (optional): specifies a class or interface to use for lazy initializing proxies.
	`lazy` (optional, defaults to `true`): setting `lazy="false"` disables the use of lazy fetching.

No discriminator column or key column is required for this mapping strategy.

For information about inheritance mappings see Chapter 9, Inheritance mapping.

`5.1.20. Join`

Using the <join> element, it is possible to map properties of one class to several tables that have a one-to-one relationship. For example:

<join
        table="tablename"                        
        schema="owner"                           
        catalog="catalog"                        
        fetch="join|select"                      
        inverse="true|false"                     
        optional="true|false">                   

        <key ... />

        <property ... />
        ...
</join>

	`table`: the name of the joined table.
	`schema` (optional): overrides the schema name specified by the root `<hibernate-mapping>` element.
	`catalog` (optional): overrides the catalog name specified by the root `<hibernate-mapping>` element.
	`fetch` (optional - defaults to `join`): if set to `join`, the default, Hibernate will use an inner join to retrieve a `<join>` defined by a class or its superclasses. It will use an outer join for a `<join>` defined by a subclass. If set to `select` then Hibernate will use a sequential select for a `<join>` defined on a subclass. This will be issued only if a row represents an instance of the subclass. Inner joins will still be used to retrieve a `<join>` defined by the class and its superclasses.
	`inverse` (optional - defaults to `false`): if enabled, Hibernate will not insert or update the properties defined by this join.
	`optional` (optional - defaults to `false`): if enabled, Hibernate will insert a row only if the properties defined by this join are non-null. It will always use an outer join to retrieve the properties.

For example, address information for a person can be mapped to a separate table while preserving value type semantics for all properties:

<class name="Person"
    table="PERSON">

    <id name="id" column="PERSON_ID">...</id>

    <join table="ADDRESS">
        <key column="ADDRESS_ID"/>
        <property name="address"/>
        <property name="zip"/>
        <property name="country"/>
    </join>
    ...

This feature is often only useful for legacy data models. We recommend fewer tables than classes and a fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, as explained later.

`5.1.21. Key`

The <key> element has featured a few times within this guide. It appears anywhere the parent mapping element defines a join to a new table that references the primary key of the original table. It also defines the foreign key in the joined table:

<key
        column="columnname"                      
        on-delete="noaction|cascade"             
        property-ref="propertyName"              
        not-null="true|false"                    
        update="true|false"                      
        unique="true|false"                      
/>

	`column` (optional): the name of the foreign key column. This can also be specified by nested `<column>` element(s).
	`on-delete` (optional - defaults to `noaction`): specifies whether the foreign key constraint has database-level cascade delete enabled.
	`property-ref` (optional): specifies that the foreign key refers to columns that are not the primary key of the original table. It is provided for legacy data.
	`not-null` (optional): specifies that the foreign key columns are not nullable. This is implied whenever the foreign key is also part of the primary key.
	`update` (optional): specifies that the foreign key should never be updated. This is implied whenever the foreign key is also part of the primary key.
	`unique` (optional): specifies that the foreign key should have a unique constraint. This is implied whenever the foreign key is also the primary key.

For systems where delete performance is important, we recommend that all keys should be defined on-delete="cascade". Hibernate uses a database-level ON CASCADE DELETE constraint, instead of many individual DELETE statements. Be aware that this feature bypasses Hibernate's usual optimistic locking strategy for versioned data.

The not-null and update attributes are useful when mapping a unidirectional one-to-many association. If you map a unidirectional one-to-many association to a non-nullable foreign key, you must declare the key column using <key not-null="true">.

`5.1.22. Column and formula elements`

Mapping elements which accept a column attribute will alternatively accept a <column> subelement. Likewise, <formula> is an alternative to the formula attribute. For example:

<column
        name="column_name"
        length="N"
        precision="N"
        scale="N"
        not-null="true|false"
        unique="true|false"
        unique-key="multicolumn_unique_key_name"
        index="index_name"
        sql-type="sql_type_name"
        check="SQL expression"
        default="SQL expression"/>

<formula>SQL expression</formula>

column and formula attributes can even be combined within the same property or association mapping to express, for example, exotic join conditions.

<many-to-one name="homeAddress" class="Address"
        insert="false" update="false">
    <column name="person_id" not-null="true" length="10"/>
    <formula>'MAILING'</formula>
</many-to-one>

`5.1.23. Import`

If your application has two persistent classes with the same name, and you do not want to specify the fully qualified package name in Hibernate queries, classes can be "imported" explicitly, rather than relying upon auto-import="true". You can also import classes and interfaces that are not explicitly mapped:

<import class="java.lang.Object" rename="Universe"/>

<import
        class="ClassName"              
        rename="ShortName"             
/>

	`class`: the fully qualified class name of any Java class.
	`rename` (optional - defaults to the unqualified class name): a name that can be used in the query language.

`5.1.24. Any`

There is one more type of property mapping. The <any> mapping element defines a polymorphic association to classes from multiple tables. This type of mapping requires more than one column. The first column contains the type of the associated entity. The remaining columns contain the identifier. It is impossible to specify a foreign key constraint for this kind of association. This is not the usual way of mapping polymorphic associations and you should use this only in special cases. For example, for audit logs, user session data, etc.

The meta-type attribute allows the application to specify a custom type that maps database column values to persistent classes that have identifier properties of the type specified by id-type. You must specify the mapping from values of the meta-type to class names.

<any name="being" id-type="long" meta-type="string">
    <meta-value value="TBL_ANIMAL" class="Animal"/>
    <meta-value value="TBL_HUMAN" class="Human"/>
    <meta-value value="TBL_ALIEN" class="Alien"/>
    <column name="table_name"/>
    <column name="id"/>
</any>

<any
        name="propertyName"                      
        id-type="idtypename"                     
        meta-type="metatypename"                 
        cascade="cascade_style"                  
        access="field|property|ClassName"        
        optimistic-lock="true|false"             
>
        <meta-value ... />
        <meta-value ... />
        .....
        <column .... />
        <column .... />
        .....
</any>

	`name`: the property name.
	`id-type`: the identifier type.
	`meta-type` (optional - defaults to `string`): any type that is allowed for a discriminator mapping.
	`cascade` (optional- defaults to `none`): the cascade style.
	`access` (optional - defaults to `property`): the strategy Hibernate uses for accessing the property value.
	`optimistic-lock` (optional - defaults to `true`): specifies that updates to this property either do or do not require acquisition of the optimistic lock. It defines whether a version increment should occur if this property is dirty.

`5.2. Hibernate types`

<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
    <column name="first_string"/>
    <column name="second_string"/>
</property>

Notice the use of <column> tags to map a property to multiple columns.

The CompositeUserType, EnhancedUserType, UserCollectionType, and UserVersionType interfaces provide support for more specialized uses.

You can even supply parameters to a UserType in the mapping file. To do this, your UserType must implement the org.hibernate.usertype.ParameterizedType interface. To supply parameters to your custom type, you can use the <type> element in your mapping files.

<property name="priority">
    <type name="com.mycompany.usertypes.DefaultValueIntegerType">
        <param name="default">0</param>
    </type>
</property>

The UserType can now retrieve the value for the parameter named default from the Properties object passed to it.

If you regularly use a certain UserType, it is useful to define a shorter name for it. You can do this using the <typedef> element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type is parameterized.

<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero">
    <param name="default">0</param>
</typedef>

<property name="priority" type="default_zero"/>

It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping.

Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use a custom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your application. For example, a MonetaryAmount class is a good candidate for a CompositeUserType, even though it could be mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would be protected against changes to the way monetary values are represented.

`5.3. Mapping a class more than once`

It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify an entity name to disambiguate between instances of the two mapped entities. By default, the entity name is the same as the class name. Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or when mapping associations to the named entity.

<class name="Contract" table="Contracts"
        entity-name="CurrentContract">
    ...
    <set name="history" inverse="true"
            order-by="effectiveEndDate desc">
        <key column="currentContractId"/>
        <one-to-many entity-name="HistoricalContract"/>
    </set>
</class>

<class name="Contract" table="ContractHistory"
        entity-name="HistoricalContract">
    ...
    <many-to-one name="currentContract"
            column="currentContractId"
            entity-name="CurrentContract"/>
</class>

Associations are now specified using entity-name instead of class.

`5.4. SQL quoted identifiers`

You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks in the mapping document. Hibernate will use the correct quotation style for the SQL Dialect. This is usually double quotes, but the SQL Server uses brackets and MySQL uses backticks.

<class name="LineItem" table="`Line Item`">
    <id name="id" column="`Item Id`"/><generator class="assigned"/></id>
    <property name="itemNumber" column="`Item #`"/>
    ...
</class>

`5.5. Metadata alternatives`

XML does not suit all users so there are some alternative ways to define O/R mapping metadata in Hibernate.

`5.5.1. Using XDoclet markup`

Many Hibernate users prefer to embed mapping information directly in sourcecode using XDoclet @hibernate.tags. We do not cover this approach in this reference guide since it is considered part of XDoclet. However, we include the following example of the Cat class with XDoclet mappings:

package eg;
import java.util.Set;
import java.util.Date;

/**
 * @hibernate.class
 *  table="CATS"
 */
public class Cat {
    private Long id; // identifier
    private Date birthdate;
    private Cat mother;
    private Set kittens
    private Color color;
    private char sex;
    private float weight;

    /*
     * @hibernate.id
     *  generator-class="native"
     *  column="CAT_ID"
     */
    public Long getId() {
        return id;
    }
    private void setId(Long id) {
        this.id=id;
    }

    /**
     * @hibernate.many-to-one
     *  column="PARENT_ID"
     */
    public Cat getMother() {
        return mother;
    }
    void setMother(Cat mother) {
        this.mother = mother;
    }

    /**
     * @hibernate.property
     *  column="BIRTH_DATE"
     */
    public Date getBirthdate() {
        return birthdate;
    }
    void setBirthdate(Date date) {
        birthdate = date;
    }
    /**
     * @hibernate.property
     *  column="WEIGHT"
     */
    public float getWeight() {
        return weight;
    }
    void setWeight(float weight) {
        this.weight = weight;
    }

    /**
     * @hibernate.property
     *  column="COLOR"
     *  not-null="true"
     */
    public Color getColor() {
        return color;
    }
    void setColor(Color color) {
        this.color = color;
    }
    /**
     * @hibernate.set
     *  inverse="true"
     *  order-by="BIRTH_DATE"
     * @hibernate.collection-key
     *  column="PARENT_ID"
     * @hibernate.collection-one-to-many
     */
    public Set getKittens() {
        return kittens;
    }
    void setKittens(Set kittens) {
        this.kittens = kittens;
    }
    // addKitten not needed by Hibernate
    public void addKitten(Cat kitten) {
        kittens.add(kitten);
    }

    /**
     * @hibernate.property
     *  column="SEX"
     *  not-null="true"
     *  update="false"
     */
    public char getSex() {
        return sex;
    }
    void setSex(char sex) {
        this.sex=sex;
    }
}

See the Hibernate website for more examples of XDoclet and Hibernate.

`5.5.2. Using JDK 5.0 Annotations`

JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe and checked at compile time. This mechanism is more powerful than XDoclet annotations and better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification (JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans. Hibernate3 implements the EntityManager of JSR-220 (the persistence API). Support for mapping metadata is available via the Hibernate Annotations package as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.

This is an example of a POJO class annotated as an EJB entity bean:

@Entity(access = AccessType.FIELD)
public class Customer implements Serializable {

    @Id;
    Long id;

    String firstName;
    String lastName;
    Date birthday;

    @Transient
    Integer age;

    @Embedded
    private Address homeAddress;

    @OneToMany(cascade=CascadeType.ALL)
    @JoinColumn(name="CUSTOMER_ID")
    Set<Order> orders;

    // Getter/setter and business methods
}

`Note`

Support for JDK 5.0 Annotations (and JSR-220) is currently under development. Please refer to the Hibernate Annotations module for more details.

`5.6. Generated properties`

Generated properties are properties that have their values generated by the database. Typically, Hibernate applications needed to refresh objects that contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. When Hibernate issues an SQL INSERT or UPDATE for an entity that has defined generated properties, it immediately issues a select afterwards to retrieve the generated values.

Properties marked as generated must additionally be non-insertable and non-updateable. Only versions, timestamps, and simple properties, can be marked as generated.

never (the default): the given property value is not generated within the database.

insert: the given property value is generated on insert, but is not regenerated on subsequent updates. Properties like created-date fall into this category. Even though version and timestamp properties can be marked as generated, this option is not available.

always: the property value is generated both on insert and on update.

`5.7. Auxiliary database objects`

Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with Hibernate's schema evolution tools, they have the ability to fully define a user schema within the Hibernate mapping files. Although designed specifically for creating and dropping things like triggers or stored procedures, any SQL command that can be run via a java.sql.Statement.execute() method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for defining auxiliary database objects:

The first mode is to explicitly list the CREATE and DROP commands in the mapping file:

<hibernate-mapping>
    ...
    <database-object>
        <create>CREATE TRIGGER my_trigger ...</create>
        <drop>DROP TRIGGER my_trigger</drop>
    </database-object>
</hibernate-mapping>

The second mode is to supply a custom class that constructs the CREATE and DROP commands. This custom class must implement the org.hibernate.mapping.AuxiliaryDatabaseObject interface.

<hibernate-mapping>
    ...
    <database-object>
        <definition class="MyTriggerDefinition"/>
    </database-object>
</hibernate-mapping>

Additionally, these database objects can be optionally scoped so that they only apply when certain dialects are used.

<hibernate-mapping>
    ...
    <database-object>
        <definition class="MyTriggerDefinition"/>
        <dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
        <dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
    </database-object>
</hibernate-mapping>