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Improved formatting and links of migration docs.
This commit is contained in:
Tim Graham
parent
8905fcbda6
commit
ab8d8e00c9
@ -2,39 +2,39 @@
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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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.. module:: django.db.migrations.operations
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Migration files are composed of one or more ``Operation``\s, 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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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 specialized Operation objects which are for things like
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There are also more specialized ``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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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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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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migration autodetector and make resulting runs of :djadmin:`makemigrations`
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output 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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.. class:: 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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@ -53,21 +53,17 @@ it can contain both class objects as well as strings in the format
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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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.. class:: 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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.. class:: RenameModel(old_name, new_name)
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Renames the model from an old name to a new one.
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@ -77,41 +73,36 @@ 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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.. class:: AlterModelTable(name, table)
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Changes the model's table name (the :attr:`~django.db.models.Options.db_table`
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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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.. class:: AlterUniqueTogether(name, unique_together)
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Changes the model's set of unique constraints (the
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:attr:`~django.db.models.Options.unique_together` option on the ``Meta``
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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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.. class:: AlterIndexTogether(name, index_together)
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Changes the model's set of custom indexes (the
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:attr:`~django.db.models.Options.index_together` option on the ``Meta``
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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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.. class:: 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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@ -126,12 +117,10 @@ 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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.. class:: RemoveField(model_name, name)
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Removes a field from a model.
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@ -139,42 +128,34 @@ 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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.. class:: 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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Alters a field's definition, including changes to its type,
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:attr:`~django.db.models.Field.null`, :attr:`~django.db.models.Field.unique`,
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:attr:`~django.db.models.Field.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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.. class:: RenameField(model_name, old_name, new_name)
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Changes a field's name (and, unless :attr:`~django.db.models.Field.db_column`
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is set, its column name).
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Special Operations
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==================
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.. _operation-run-sql:
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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)
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.. class:: RunSQL(sql, reverse_sql=None, state_operations=None)
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Allows running 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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@ -194,24 +175,22 @@ operation that adds that field and so will try to run it again).
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.. _sqlparse: https://pypi.python.org/pypi/sqlparse
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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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.. class:: 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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instance of :class:`SchemaEditor
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<django.db.backends.schema.BaseDatabaseSchemaEditor>`.
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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``. Here's an
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example of using RunPython to create some initial objects on a Country model::
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example of using ``RunPython`` to create some initial objects on a ``Country``
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model::
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# encoding: utf8
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from django.db import models, migrations
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@ -245,19 +224,16 @@ or ``orm["appname", "Model"]`` references from South directly into
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``apps.get_model("appname", "Model")`` references here and leave most of the
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rest of the code unchanged for data migrations.
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Much like ``RunSQL``, ensure that if you change schema inside here you're
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Much like :class:`RunSQL`, ensure that if you change schema inside here you're
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either doing it outside the scope of the Django model system (e.g. triggers)
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or that you use ``SeparateDatabaseAndState`` to add in operations that will
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or that you use :class:`SeparateDatabaseAndState` to add in operations that will
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reflect your changes to the model state - otherwise, the versioned ORM and
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the autodetector will stop working correctly.
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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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.. class:: SeparateDatabaseAndState(database_operations=None, state_operations=None)
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A highly specialized 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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@ -266,13 +242,12 @@ 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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of an ``Operation`` looks like this::
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from django.db.migrations.operations.base import Operation
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@ -317,7 +292,7 @@ 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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* 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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@ -1,6 +1,10 @@
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============
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SchemaEditor
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============
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================
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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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@ -27,10 +31,10 @@ 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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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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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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@ -41,9 +45,7 @@ Methods
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execute
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-------
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::
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execute(sql, params=[])
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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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@ -52,92 +54,71 @@ 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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::
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create_model(model)
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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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::
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delete_model(model)
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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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::
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alter_unique_together(model, old_unique_together, new_unique_together)
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Changes a model's unique_together value; this will add or remove unique
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constraints from the model's table until they match the new value.
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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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::
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alter_index_together(model, old_index_together, new_index_together)
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Changes a model's index_together value; this will add or remove indexes
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from the model's table until they match the new value.
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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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::
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alter_db_table(model, old_db_table, new_db_table)
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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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::
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alter_db_tablespace(model, old_db_tablespace, new_db_tablespace)
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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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::
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add_field(model, field)
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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 of
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creating a column, it will make a table to represent the relationship. If
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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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::
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remove_field(model, field)
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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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@ -147,25 +128,22 @@ 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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::
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alter_field(model, old_field, new_field, strict=False)
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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 ``db_column`` attribute),
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changing the type of the field (if the field class changes), changing
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the ``NULL`` status of the field, adding or removing field-only unique
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constraints and indexes, changing primary key, and changing the destination
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of ForeignKey constraints.
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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, ``remove_field``
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and ``add_field`` should be called separately.
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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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@ -639,8 +639,9 @@ Management Commands
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* :ref:`initial-sql` now works better if the sqlparse_ Python library is
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installed.
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Note that it's deprecated in favor of the :ref:`RunSQL <operation-run-sql>`
|
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operation of migrations, which benefits from the improved behavior.
|
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Note that it's deprecated in favor of the
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:class:`~django.db.migrations.operations.RunSQL` operation of migrations,
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which benefits from the improved behavior.
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.. _sqlparse: https://pypi.python.org/pypi/sqlparse
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@ -17,7 +17,7 @@ A Brief History
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Prior to version 1.7, Django only supported adding new models to the
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database; it was not possible to alter or remove existing models via the
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``syncdb`` command (the predecessor to ``migrate``).
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``syncdb`` command (the predecessor to :djadmin:`migrate`).
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Third-party tools, most notably `South <http://south.aeracode.org>`_,
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provided support for these additional types of change, but it was considered
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@ -53,7 +53,8 @@ staging machines, and eventually your production machines.
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.. note::
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It is possible to override the name of the package which contains the
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migrations on a per-app basis by modifying the :setting:`MIGRATION_MODULES` setting.
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migrations on a per-app basis by modifying the :setting:`MIGRATION_MODULES`
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setting.
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Migrations will run the same way on the same dataset and produce consistent
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results, meaning that what you see in development and staging is, under the
|
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@ -184,14 +185,14 @@ Dependencies
|
||||
While migrations are per-app, the tables and relationships implied by
|
||||
your models are too complex to be created for just one app at a time. When
|
||||
you make a migration that requires something else to run - for example,
|
||||
you add a ForeignKey in your ``books`` app to your ``authors`` app - the
|
||||
you add a ``ForeignKey`` in your ``books`` app to your ``authors`` app - the
|
||||
resulting migration will contain a dependency on a migration in ``authors``.
|
||||
|
||||
This means that when you run the migrations, the ``authors`` migration runs
|
||||
first and creates the table the ``ForeignKey`` references, and then the migration
|
||||
that makes the ``ForeignKey`` column runs afterwards and creates the constraint.
|
||||
If this didn't happen, the migration would try to create the ForeignKey column
|
||||
without the table it's referencing existing and your database would
|
||||
If this didn't happen, the migration would try to create the ``ForeignKey``
|
||||
column without the table it's referencing existing and your database would
|
||||
throw an error.
|
||||
|
||||
This dependency behavior affects most migration operations where you
|
||||
@ -228,8 +229,8 @@ inspects this object for four attributes, only two of which are used
|
||||
most of the time:
|
||||
|
||||
* ``dependencies``, a list of migrations this one depends on.
|
||||
* ``operations``, a list of Operation classes that define what this migration
|
||||
does.
|
||||
* ``operations``, a list of ``Operation`` classes that define what this
|
||||
migration does.
|
||||
|
||||
The operations are the key; they are a set of declarative instructions which
|
||||
tell Django what schema changes need to be made. Django scans them and
|
||||
@ -252,9 +253,9 @@ Custom fields
|
||||
~~~~~~~~~~~~~
|
||||
|
||||
You can't modify the number of positional arguments in an already migrated
|
||||
custom field without raising a TypeError. The old migration will call the
|
||||
custom field without raising a ``TypeError``. The old migration will call the
|
||||
modified ``__init__`` method with the old signature. So if you need a new
|
||||
argument, please create a keyword argument and use e.g.
|
||||
argument, please create a keyword argument and add something like
|
||||
``assert kwargs.get('argument_name') is not None`` in the constructor.
|
||||
|
||||
Adding migrations to apps
|
||||
@ -285,7 +286,6 @@ Note that this only works given two things:
|
||||
that your database doesn't match your models, you'll just get errors when
|
||||
migrations try to modify those tables.
|
||||
|
||||
|
||||
.. _historical-models:
|
||||
|
||||
Historical models
|
||||
@ -293,9 +293,9 @@ Historical models
|
||||
|
||||
When you run migrations, Django is working from historical versions of
|
||||
your models stored in the migration files. If you write Python code
|
||||
using the ``django.db.migrations.RunPython`` operation, or if you have
|
||||
``allow_migrate`` methods on your database routers, you will be exposed
|
||||
to these versions of your models.
|
||||
using the :class:`~django.db.migrations.operations.RunPython` operation, or if
|
||||
you have ``allow_migrate`` methods on your database routers, you will be
|
||||
exposed to these versions of your models.
|
||||
|
||||
Because it's impossible to serialize arbitrary Python code, these historical
|
||||
models will not have any custom methods or managers that you have defined.
|
||||
@ -304,9 +304,9 @@ They will, however, have the same fields, relationships and ``Meta`` options
|
||||
|
||||
.. warning::
|
||||
|
||||
This means that you will NOT have custom save() methods called on objects
|
||||
when you access them in migrations, and you will NOT have any custom constructors
|
||||
or instance methods. Plan appropriately!
|
||||
This means that you will NOT have custom ``save()`` methods called on objects
|
||||
when you access them in migrations, and you will NOT have any custom
|
||||
constructors or instance methods. Plan appropriately!
|
||||
|
||||
In addition, the base classes of the model are just stored as pointers,
|
||||
so you must always keep base classes around for as long as there is a migration
|
||||
@ -314,7 +314,6 @@ that contains a reference to them. On the plus side, methods and managers
|
||||
from these base classes inherit normally, so if you absolutely need access
|
||||
to these you can opt to move them into a superclass.
|
||||
|
||||
|
||||
.. _data-migrations:
|
||||
|
||||
Data Migrations
|
||||
@ -330,7 +329,7 @@ Django can't automatically generate data migrations for you, as it does with
|
||||
schema migrations, but it's not very hard to write them. Migration files in
|
||||
Django are made up of :doc:`Operations </ref/migration-operations>`, and
|
||||
the main operation you use for data migrations is
|
||||
:ref:`RunPython <operation-run-python>`.
|
||||
:class:`~django.db.migrations.operations.RunPython`.
|
||||
|
||||
To start, make an empty migration file you can work from (Django will put
|
||||
the file in the right place, suggest a name, and add dependencies for you)::
|
||||
@ -351,13 +350,15 @@ Then, open up the file; it should look something like this::
|
||||
operations = [
|
||||
]
|
||||
|
||||
Now, all you need to do is create a new function and have RunPython use it.
|
||||
RunPython expects a callable as its argument which takes two arguments - the
|
||||
first is an :doc:`app registry </ref/applications/>` that has the historical
|
||||
versions of all your models loaded into it to match where in your history the
|
||||
migration sits, and the second is a :doc:`SchemaEditor </ref/schema-editor>`,
|
||||
which you can use to manually effect database schema changes (but beware,
|
||||
doing this can confuse the migration autodetector!)
|
||||
Now, all you need to do is create a new function and have
|
||||
:class:`~django.db.migrations.operations.RunPython` use it.
|
||||
:class:`~django.db.migrations.operations.RunPython` expects a callable as its argument
|
||||
which takes two arguments - the first is an :doc:`app registry
|
||||
</ref/applications/>` that has the historical versions of all your models
|
||||
loaded into it to match where in your history the migration sits, and the
|
||||
second is a :doc:`SchemaEditor </ref/schema-editor>`, which you can use to
|
||||
manually effect database schema changes (but beware, doing this can confuse
|
||||
the migration autodetector!)
|
||||
|
||||
Let's write a simple migration that populates our new ``name`` field with the
|
||||
combined values of ``first_name`` and ``last_name`` (we've come to our senses
|
||||
@ -389,8 +390,8 @@ Once that's done, we can just run ``python manage.py migrate`` as normal and
|
||||
the data migration will run in place alongside other migrations.
|
||||
|
||||
If you're interested in the more advanced migration operations, or want
|
||||
to be able to write your own, see our
|
||||
:doc:`migration operations reference </ref/migration-operations>`.
|
||||
to be able to write your own, see the :doc:`migration operations reference
|
||||
</ref/migration-operations>`.
|
||||
|
||||
.. _migration-squashing:
|
||||
|
||||
@ -406,15 +407,19 @@ Squashing is the act of reducing an existing set of many migrations down to
|
||||
one (or sometimes a few) migrations which still represent the same changes.
|
||||
|
||||
Django does this by taking all of your existing migrations, extracting their
|
||||
Operations and putting them all in sequence, and then running an optimizer
|
||||
``Operation``\s and putting them all in sequence, and then running an optimizer
|
||||
over them to try and reduce the length of the list - for example, it knows
|
||||
that ``CreateModel`` and ``DeleteModel`` cancel each other out, and it knows
|
||||
that ``AddColumn`` can be rolled into ``CreateModel``.
|
||||
that :class:`~django.db.migrations.operations.CreateModel` and
|
||||
:class:`~django.db.migrations.operations.DeleteModel` cancel each other out,
|
||||
and it knows that :class:`~django.db.migrations.operations.AddField` can be
|
||||
rolled into :class:`~django.db.migrations.operations.CreateModel`.
|
||||
|
||||
Once the operation sequence has been reduced as much as possible - the amount
|
||||
possible depends on how closely intertwined your models are and if you have
|
||||
any RunSQL or RunPython operations (which can't be optimized through) - Django
|
||||
will them write it back out into a new set of initial migration files.
|
||||
any :class:`~django.db.migrations.operations.RunSQL`
|
||||
or :class:`~django.db.migrations.operations.RunPython` operations (which can't
|
||||
be optimized through) - Django will them write it back out into a new set of
|
||||
initial migration files.
|
||||
|
||||
These files are marked to say they replace the previously-squashed migrations,
|
||||
so they can coexist with the old migration files, and Django will intelligently
|
||||
@ -452,9 +457,9 @@ work::
|
||||
Note that model interdependencies in Django can get very complex, and squashing
|
||||
may occasionally result in an optimized migration that doesn't work or is
|
||||
impossible to run. When this occurs, you can re-try with ``--no-optimize``, but
|
||||
please file a bug report either way detailing the models and their
|
||||
relationships so we can improve the optimizer to handle your case.
|
||||
|
||||
please `file a bug report <https://code.djangoproject.com/newticket>`_ either
|
||||
way detailing the models and their relationships so we can improve the
|
||||
optimizer to handle your case.
|
||||
|
||||
.. _migration-serializing:
|
||||
|
||||
@ -508,7 +513,6 @@ available at the top level of a module it is not serializable.
|
||||
Django will write out the value as an instantiation of your class with the
|
||||
given arguments, similar to the way it writes out references to Django fields.
|
||||
|
||||
|
||||
Upgrading from South
|
||||
--------------------
|
||||
|
||||
@ -517,9 +521,13 @@ If you already have pre-existing migrations created with
|
||||
``django.db.migrations`` is quite simple:
|
||||
|
||||
* Ensure all installs are fully up-to-date with their migrations
|
||||
* Delete all your (numbered) migration files, but not the directory or __init__.py - make sure you remove the ``.pyc`` files too.
|
||||
* Run ``python manage.py makemigrations``. Django should see the empty migration directories and make new initial migrations in the new format.
|
||||
* Run ``python manage.py migrate``. Django will see that the tables for the initial migrations already exist and mark them as applied without running them.
|
||||
* Delete all your (numbered) migration files, but not the directory or
|
||||
``__init__.py`` - make sure you remove the ``.pyc`` files too.
|
||||
* Run ``python manage.py makemigrations``. Django should see the empty
|
||||
migration directories and make new initial migrations in the new format.
|
||||
* Run ``python manage.py migrate``. Django will see that the tables for the
|
||||
initial migrations already exist and mark them as applied without running
|
||||
them.
|
||||
|
||||
That's it! The only complication is if you have a circular dependency loop
|
||||
of foreign keys; in this case, ``makemigrations`` might make more than one
|
||||
|
||||
Loading…
Reference in New Issue
Block a user