SchemaEditor
¶Django’s migration system is split into two parts; the logic for calculating
and storing what operations should be run (django.db.migrations
), and the
database abstraction layer that turns things like “create a model” or
“delete a field” into SQL - which is the job of the SchemaEditor
.
It’s unlikely that you will want to interact directly with SchemaEditor
as
a normal developer using Django, but if you want to write your own migration
system, or have more advanced needs, it’s a lot nicer than writing SQL.
Each database backend in Django supplies its own version of SchemaEditor
,
and it’s always accessible via the connection.schema_editor()
context
manager:
with connection.schema_editor() as schema_editor:
schema_editor.delete_model(MyModel)
It must be used via the context manager as this allows it to manage things
like transactions and deferred SQL (like creating ForeignKey
constraints).
It exposes all possible operations as methods, that should be called in the order you wish changes to be applied. Some possible operations or types of change are not possible on all databases - for example, MyISAM does not support foreign key constraints.
If you are writing or maintaining a third-party database backend for Django,
you will need to provide a SchemaEditor
implementation in order to work with
1.7’s migration functionality - however, as long as your database is relatively
standard in its use of SQL and relational design, you should be able to
subclass one of the built-in Django SchemaEditor
classes and just tweak the
syntax a little. Also note that there are a few new database features that
migrations will look for: can_rollback_ddl
and
supports_combined_alters
are the most important.
execute()
¶Executes the SQL statement passed in, with parameters if supplied. This
is a simple wrapper around the normal database cursors that allows
capture of the SQL to a .sql
file if the user wishes.
create_model()
¶Creates a new table in the database for the provided model, along with any unique constraints or indexes it requires.
delete_model()
¶Drops the model’s table in the database along with any unique constraints or indexes it has.
alter_unique_together()
¶BaseDatabaseSchemaEditor.
alter_unique_together
(model, old_unique_together, new_unique_together)[source]¶Changes a model’s unique_together
value; this
will add or remove unique constraints from the model’s table until they match
the new value.
alter_index_together()
¶BaseDatabaseSchemaEditor.
alter_index_together
(model, old_index_together, new_index_together)[source]¶Changes a model’s index_together
value; this
will add or remove indexes from the model’s table until they match the new
value.
alter_db_table()
¶Renames the model’s table from old_db_table
to new_db_table
.
alter_db_tablespace()
¶Moves the model’s table from one tablespace to another.
add_field()
¶Adds a column (or sometimes multiple) to the model’s table to represent the
field. This will also add indexes or a unique constraint
if the field has db_index=True
or unique=True
.
If the field is a ManyToManyField
without a value for through
, instead
of creating a column, it will make a table to represent the relationship. If
through
is provided, it is a no-op.
If the field is a ForeignKey
, this will also add the foreign key
constraint to the column.
remove_field()
¶Removes the column(s) representing the field from the model’s table, along with any unique constraints, foreign key constraints, or indexes caused by that field.
If the field is a ManyToManyField without a value for through
, it will
remove the table created to track the relationship. If
through
is provided, it is a no-op.
alter_field()
¶This transforms the field on the model from the old field to the new one. This
includes changing the name of the column (the
db_column
attribute), changing the type of the
field (if the field class changes), changing the NULL
status of the field,
adding or removing field-only unique constraints and indexes, changing primary
key, and changing the destination of ForeignKey
constraints.
The most common transformation this cannot do is transforming a
ManyToManyField
into a normal Field or vice-versa; Django cannot do this
without losing data, and so it will refuse to do it. Instead,
remove_field()
and add_field()
should be called separately.
If the database has the supports_combined_alters
, Django will try and
do as many of these in a single database call as possible; otherwise, it will
issue a separate ALTER statement for each change, but will not issue ALTERs
where no change is required (as South often did).
All attributes should be considered read-only unless stated otherwise.
connection
¶SchemaEditor.
connection
¶A connection object to the database. A useful attribute of the connection is
alias
which can be used to determine the name of the database being
accessed.
This is useful when doing data migrations for migrations with multiple databases.
Jun 09, 2017