Viewing github.com/django/django-stable-4.1.x/docs/ref/schema-editor.txt

.. module:: django.db.backends.base.schema

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

    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

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

Django'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 tweak the

.. method:: BaseDatabaseSchemaEditor.execute(sql, params=())

Executes the SQL statement passed in, with parameters if supplied. This

is a wrapper around the normal database cursors that allows capture of the SQL

.. method:: BaseDatabaseSchemaEditor.create_model(model)

Creates a new table in the database for the provided model, along with any

unique constraints or indexes it requires.

.. method:: BaseDatabaseSchemaEditor.delete_model(model)

Drops the model's table in the database along with any unique constraints

.. method:: BaseDatabaseSchemaEditor.add_index(model, index)

.. method:: BaseDatabaseSchemaEditor.remove_index(model, index)

Removes ``index`` from ``model``’s table.

.. method:: BaseDatabaseSchemaEditor.rename_index(model, old_index, new_index)

Renames ``old_index`` from ``model``’s table to ``new_index``.

.. method:: BaseDatabaseSchemaEditor.add_constraint(model, constraint)

Adds ``constraint`` to ``model``'s table.

.. method:: BaseDatabaseSchemaEditor.remove_constraint(model, constraint)

Removes ``constraint`` from ``model``'s table.

.. method:: BaseDatabaseSchemaEditor.alter_unique_together(model, old_unique_together, new_unique_together)

Changes a model's :attr:`~django.db.models.Options.unique_together` value; this

will add or remove unique constraints from the model's table until they match

.. method:: BaseDatabaseSchemaEditor.alter_index_together(model, old_index_together, new_index_together)

Changes a model's :attr:`~django.db.models.Options.index_together` value; this

will add or remove indexes from the model's table until they match the new

.. method:: BaseDatabaseSchemaEditor.alter_db_table(model, old_db_table, new_db_table)

Renames the model's table from ``old_db_table`` to ``new_db_table``.

.. method:: BaseDatabaseSchemaEditor.alter_db_tablespace(model, old_db_tablespace, new_db_tablespace)

Moves the model's table from one tablespace to another.

.. method:: BaseDatabaseSchemaEditor.add_field(model, 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

.. method:: BaseDatabaseSchemaEditor.remove_field(model, 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

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.

.. method:: BaseDatabaseSchemaEditor.alter_field(model, old_field, new_field, strict=False)

This transforms the field on the model from the old field to the new one. This

includes changing the name of the column (the

:attr:`~django.db.models.Field.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,

:meth:`.remove_field` and :meth:`.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

All attributes should be considered read-only unless stated otherwise.

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

This is useful when doing data migrations for :ref:`migrations with multiple

databases <data-migrations-and-multiple-databases>`.