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321 lines
11 KiB
Plaintext
====================
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Migration Operations
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====================
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Migration files are composed of one or more Operations, objects that
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declaratively record what the migration should do to your database.
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Django also uses these Operation objects to work out what your models
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looked like historically, and to calculate what changes you've made to
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your models since the last migration so it can automatically write
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your migrations; that's why they're declarative, as it means Django can
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easily load them all into memory and run through them without touching
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the database to work out what your project should look like.
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There are also more specialised Operation objects which are for things like
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:ref:`data migrations <data-migrations>` and for advanced manual database
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manipulation. You can also write your own Operation classes if you want
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to encapsulate a custom change you commonly make.
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If you need an empty migration file to write your own Operation objects
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into, just use ``python manage.py makemigrations --empty yourappname``,
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but be aware that manually adding schema-altering operations can confuse the
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migration autodetector and make resulting runs of ``makemigrations`` output
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incorrect code.
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All of the core Django operations are available from the
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``django.db.migrations.operations`` module.
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Schema Operations
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=================
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CreateModel
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-----------
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::
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CreateModel(name, fields, options=None, bases=None)
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Creates a new model in the project history and a corresponding table in the
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database to match it.
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``name`` is the model name, as would be written in the ``models.py`` file.
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``fields`` is a list of 2-tuples of ``(field_name, field_instance)``.
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The field instance should be an unbound field (so just ``models.CharField()``,
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rather than a field takes from another model).
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``options`` is an optional dictionary of values from the model's ``Meta`` class.
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``bases`` is an optional list of other classes to have this model inherit from;
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it can contain both class objects as well as strings in the format
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``"appname.ModelName"`` if you want to depend on another model (so you inherit
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from the historical version). If it's not supplied, it defaults to just
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inheriting from the standard ``models.Model``.
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DeleteModel
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-----------
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::
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DeleteModel(name)
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Deletes the model from the project history and its table from the database.
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RenameModel
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-----------
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::
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RenameModel(old_name, new_name)
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Renames the model from an old name to a new one.
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You may have to manually add
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this if you change the model's name and quite a few of its fields at once; to
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the autodetector, this will look like you deleted a model with the old name
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and added a new one with a different name, and the migration it creates will
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lose any data in the old table.
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AlterModelTable
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---------------
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::
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AlterModelTable(name, table)
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Changes the model's table name (the ``db_table`` option on the ``Meta`` subclass)
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AlterUniqueTogether
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-------------------
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::
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AlterUniqueTogether(name, unique_together)
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Changes the model's set of unique constraints
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(the ``unique_together`` option on the ``Meta`` subclass)
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AlterIndexTogether
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------------------
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::
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AlterIndexTogether(name, index_together)
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Changes the model's set of custom indexes
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(the ``index_together`` option on the ``Meta`` subclass)
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AddField
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--------
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::
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AddField(model_name, name, field, preserve_default=True)
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Adds a field to a model. ``model_name`` is the model's name, ``name`` is
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the field's name, and ``field`` is an unbound Field instance (the thing
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you would put in the field declaration in ``models.py`` - for example,
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``models.IntegerField(null=True)``.
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The ``preserve_default`` argument indicates whether the field's default
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value is permanent and should be baked into the project state (``True``),
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or if it is temporary and just for this migration (``False``) - usually
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because the migration is adding a non-nullable field to a table and needs
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a default value to put into existing rows. It does not effect the behavior
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of setting defaults in the database directly - Django never sets database
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defaults, and always applies them in the Django ORM code.
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RemoveField
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-----------
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::
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RemoveField(model_name, name)
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Removes a field from a model.
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Bear in mind that when reversed this is actually adding a field to a model;
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if the field is not nullable this may make this operation irreversible (apart
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from any data loss, which of course is irreversible).
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AlterField
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----------
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::
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AlterField(model_name, name, field)
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Alters a field's definition, including changes to its type, ``null``, ``unique``,
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``db_column`` and other field attributes.
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Note that not all changes are possible on all databases - for example, you
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cannot change a text-type field like ``models.TextField()`` into a number-type
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field like ``models.IntegerField()`` on most databases.
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RenameField
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-----------
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::
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RenameField(model_name, old_name, new_name)
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Changes a field's name (and, unless ``db_column`` is set, its column name).
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Special Operations
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==================
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RunSQL
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------
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::
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RunSQL(sql, reverse_sql=None, state_operations=None, multiple=False)
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Allows runnning of arbitrary SQL on the database - useful for more advanced
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features of database backends that Django doesn't support directly, like
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partial indexes.
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``sql``, and ``reverse_sql`` if provided, should be strings of SQL to run on the
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database. They will be passed to the database as a single SQL statement unless
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``multiple`` is set to ``True``, in which case they will be split into separate
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statements manually by the operation before being passed through.
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In some extreme cases, the built-in statement splitter may not be able to split
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correctly, in which case you should manually split the SQL into multiple calls
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to ``RunSQL``.
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The ``state_operations`` argument is so you can supply operations that are
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equivalent to the SQL in terms of project state; for example, if you are
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manually creating a column, you should pass in a list containing an ``AddField``
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operation here so that the autodetector still has an up-to-date state of the
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model (otherwise, when you next run ``makemigrations``, it won't see any
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operation that adds that field and so will try to run it again).
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.. _operation-run-python:
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RunPython
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---------
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::
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RunPython(code, reverse_code=None)
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Runs custom Python code in a historical context. ``code`` (and ``reverse_code``
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if supplied) should be callable objects that accept two arguments; the first is
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an instance of ``django.apps.registry.Apps`` containing historical models that
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match the operation's place in the project history, and the second is an
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instance of SchemaEditor.
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You are advised to write the code as a separate function above the ``Migration``
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class in the migration file, and just pass it to ``RunPython``.
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SeparateDatabaseAndState
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------------------------
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::
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SeparateDatabaseAndState(database_operations=None, state_operations=None)
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A highly specalist operation that let you mix and match the database
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(schema-changing) and state (autodetector-powering) aspects of operations.
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It accepts two list of operations, and when asked to apply state will use the
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state list, and when asked to apply changes to the database will use the database
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list. Do not use this operation unless you're very sure you know what you're doing.
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Writing your own
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================
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Operations have a relatively simple API, and they're designed so that you can
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easily write your own to supplement the built-in Django ones. The basic structure
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of an Operation looks like this::
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from django.db.migrations.operations.base import Operation
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class MyCustomOperation(Operation):
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# If this is False, it means that this operation will be ignored by
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# sqlmigrate; if true, it will be run and the SQL collected for its output.
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reduces_to_sql = False
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# If this is False, Django will refuse to reverse past this operation.
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reversible = False
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def __init__(self, arg1, arg2):
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# Operations are usually instantiated with arguments in migration
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# files. Store the values of them on self for later use.
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pass
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def state_forwards(self, app_label, state):
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# The Operation should take the 'state' parameter (an instance of
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# django.db.migrations.state.ProjectState) and mutate it to match
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# any schema changes that have occurred.
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pass
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def database_forwards(self, app_label, schema_editor, from_state, to_state):
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# The Operation should use schema_editor to apply any changes it
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# wants to make to the database.
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pass
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def database_backwards(self, app_label, schema_editor, from_state, to_state):
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# If reversible is True, this is called when the operation is reversed.
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pass
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def describe(self):
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# This is used to describe what the operation does in console output.
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return "Custom Operation"
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You can take this template and work from it, though we suggest looking at the
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built-in Django operations in ``django.db.migrations.operations`` - they're
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easy to read and cover a lot of the example usage of semi-internal aspects
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of the migration framework like ``ProjectState`` and the patterns used to get
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historical models.
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Some things to note:
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* You don't need to learn too much about ProjectState to just write simple
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migrations; just know that it has a ``.render()`` method that turns it into
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an app registry (which you can then call ``get_model`` on).
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* ``database_forwards`` and ``database_backwards`` both get two states passed
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to them; these just represent the difference the ``state_forwards`` method
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would have applied, but are given to you for convenience and speed reasons.
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* ``to_state`` in the database_backwards method is the *older* state; that is,
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the one that will be the current state once the migration has finished reversing.
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* You might see implementations of ``references_model`` on the built-in
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operations; this is part of the autodetection code and does not matter for
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custom operations.
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As a simple example, let's make an operation that loads PostgreSQL extensions
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(which contain some of PostgreSQL's more exciting features). It's simple enough;
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there's no model state changes, and all it does is run one command::
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from django.db.migrations.operations.base import Operation
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class LoadExtension(Operation):
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reversible = True
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def __init__(self, name):
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self.name = name
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def state_forwards(self, app_label, state):
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pass
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def database_forwards(self, app_label, schema_editor, from_state, to_state):
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schema_editor.execute("CREATE EXTENSION IF NOT EXISTS %s" % self.name)
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def database_backwards(self, app_label, schema_editor, from_state, to_state):
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schema_editor.execute("DROP EXTENSION %s" % self.name)
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def describe(self):
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return "Creates extension %s" % self.name
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