Model _meta API

class Options[源代码]

The model _meta API is at the core of the Django ORM. It enables other parts of the system such as lookups, queries, forms, and the admin to understand the capabilities of each model. The API is accessible through the _meta attribute of each model class, which is an instance of an django.db.models.options.Options object.

Methods that it provides can be used to:

  • Retrieve all field instances of a model
  • Retrieve a single field instance of a model by name

Field access API

Retrieving a single field instance of a model by name

Options.get_field(field_name)[源代码]

Returns the field instance given a name of a field.

field_name can be the name of a field on the model, a field on an abstract or inherited model, or a field defined on another model that points to the model. In the latter case, the field_name will be (in order of preference) the related_query_name set by the user, the related_name set by the user, or the name automatically generated by Django.

Hidden fields cannot be retrieved by name.

If a field with the given name is not found a FieldDoesNotExist exception will be raised.

>>> from django.contrib.auth.models import User

# A field on the model
>>> User._meta.get_field('username')
<django.db.models.fields.CharField: username>

# A field from another model that has a relation with the current model
>>> User._meta.get_field('logentry')
<ManyToOneRel: admin.logentry>

# A non existent field
>>> User._meta.get_field('does_not_exist')
Traceback (most recent call last):
    ...
FieldDoesNotExist: User has no field named 'does_not_exist'

Retrieving all field instances of a model

Options.get_fields(include_parents=True, include_hidden=False)[源代码]

Returns a tuple of fields associated with a model. get_fields() accepts two parameters that can be used to control which fields are returned:

include_parents
True by default. Recursively includes fields defined on parent classes. If set to False, get_fields() will only search for fields declared directly on the current model. Fields from models that directly inherit from abstract models or proxy classes are considered to be local, not on the parent.
include_hidden
False by default. If set to True, get_fields() will include fields that are used to back other field's functionality. This will also include any fields that have a related_name (such as ManyToManyField, or ForeignKey) that start with a "+".
>>> from django.contrib.auth.models import User
>>> User._meta.get_fields()
(<ManyToOneRel: admin.logentry>,
 <django.db.models.fields.AutoField: id>,
 <django.db.models.fields.CharField: password>,
 <django.db.models.fields.DateTimeField: last_login>,
 <django.db.models.fields.BooleanField: is_superuser>,
 <django.db.models.fields.CharField: username>,
 <django.db.models.fields.CharField: first_name>,
 <django.db.models.fields.CharField: last_name>,
 <django.db.models.fields.EmailField: email>,
 <django.db.models.fields.BooleanField: is_staff>,
 <django.db.models.fields.BooleanField: is_active>,
 <django.db.models.fields.DateTimeField: date_joined>,
 <django.db.models.fields.related.ManyToManyField: groups>,
 <django.db.models.fields.related.ManyToManyField: user_permissions>)

# Also include hidden fields.
>>> User._meta.get_fields(include_hidden=True)
(<ManyToOneRel: auth.user_groups>,
 <ManyToOneRel: auth.user_user_permissions>,
 <ManyToOneRel: admin.logentry>,
 <django.db.models.fields.AutoField: id>,
 <django.db.models.fields.CharField: password>,
 <django.db.models.fields.DateTimeField: last_login>,
 <django.db.models.fields.BooleanField: is_superuser>,
 <django.db.models.fields.CharField: username>,
 <django.db.models.fields.CharField: first_name>,
 <django.db.models.fields.CharField: last_name>,
 <django.db.models.fields.EmailField: email>,
 <django.db.models.fields.BooleanField: is_staff>,
 <django.db.models.fields.BooleanField: is_active>,
 <django.db.models.fields.DateTimeField: date_joined>,
 <django.db.models.fields.related.ManyToManyField: groups>,
 <django.db.models.fields.related.ManyToManyField: user_permissions>)

Migrating from the old API

As part of the formalization of the Model._meta API (from the django.db.models.options.Options class), a number of methods and properties have been deprecated and will be removed in Django 1.10.

These old APIs can be replicated by either:

Although it's possible to make strictly equivalent replacements of the old methods, that might not be the best approach. Taking the time to refactor any field loops to make better use of the new API - and possibly include fields that were previously excluded - will almost certainly result in better code.

Assuming you have a model named MyModel, the following substitutions can be made to convert your code to the new API:

  • MyModel._meta.get_field(name) becomes:

    f = MyModel._meta.get_field(name)

    then check if:

    • f.auto_created == False, because the new get_field() API will find "reverse" relations, and:
    • f.is_relation and f.related_model is None, because the new get_field() API will find GenericForeignKey relations.

  • MyModel._meta.get_field_by_name(name) returns a tuple of these four values with the following replacements:

    • field can be found by MyModel._meta.get_field(name)

    • model can be found through the model attribute on the field.

    • direct can be found by: not field.auto_created or field.concrete

      The auto_created check excludes all "forward" and "reverse" relations that are created by Django, but this also includes AutoField and OneToOneField on proxy models. We avoid filtering out these attributes using the concrete attribute.

    • m2m can be found through the many_to_many attribute on the field.

  • MyModel._meta.get_fields_with_model() becomes:

    [
    (f, f.model if f.model != MyModel else None)
    for f in MyModel._meta.get_fields()
    if not f.is_relation
        or f.one_to_one
        or (f.many_to_one and f.related_model)
]

  • MyModel._meta.get_concrete_fields_with_model() becomes:

    [
    (f, f.model if f.model != MyModel else None)
    for f in MyModel._meta.get_fields()
    if f.concrete and (
        not f.is_relation
        or f.one_to_one
        or (f.many_to_one and f.related_model)
    )
]

  • MyModel._meta.get_m2m_with_model() becomes:

    [
    (f, f.model if f.model != MyModel else None)
    for f in MyModel._meta.get_fields()
    if f.many_to_many and not f.auto_created
]

  • MyModel._meta.get_all_related_objects() becomes:

    [
    f for f in MyModel._meta.get_fields()
    if (f.one_to_many or f.one_to_one)
    and f.auto_created and not f.concrete
]

  • MyModel._meta.get_all_related_objects_with_model() becomes:

    [
    (f, f.model if f.model != MyModel else None)
    for f in MyModel._meta.get_fields()
    if (f.one_to_many or f.one_to_one)
    and f.auto_created and not f.concrete
]

  • MyModel._meta.get_all_related_many_to_many_objects() becomes:

    [
    f for f in MyModel._meta.get_fields(include_hidden=True)
    if f.many_to_many and f.auto_created
]

  • MyModel._meta.get_all_related_m2m_objects_with_model() becomes:

    [
    (f, f.model if f.model != MyModel else None)
    for f in MyModel._meta.get_fields(include_hidden=True)
    if f.many_to_many and f.auto_created
]

  • MyModel._meta.get_all_field_names() becomes:

    from itertools import chain
list(set(chain.from_iterable(
    (field.name, field.attname) if hasattr(field, 'attname') else (field.name,)
    for field in MyModel._meta.get_fields()
    # For complete backwards compatibility, you may want to exclude
    # GenericForeignKey from the results.
    if not (field.many_to_one and field.related_model is None)
)))

    This provides a 100% backwards compatible replacement, ensuring that both field names and attribute names ForeignKeys are included, but fields associated with GenericForeignKeys are not. A simpler version would be:

    [f.name for f in MyModel._meta.get_fields()]

    While this isn't 100% backwards compatible, it may be sufficient in many situations.

« previous | up | next »