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