===========================
Writing database migrations
===========================

This document explains how to structure and write database migrations for
different scenarios you might encounter. For introductory material on
migrations, see :doc:`the topic guide </topics/migrations>`.

.. _data-migrations-and-multiple-databases:

Data migrations and multiple databases
======================================

When using multiple databases, you may need to figure out whether or not to
run a migration against a particular database. For example, you may want to
**only** run a migration on a particular database.

In order to do that you can check the database connection's alias inside a
``RunPython`` operation by looking at the ``schema_editor.connection.alias``
attribute::

    from django.db import migrations

    def forwards(apps, schema_editor):
        if not schema_editor.connection.alias == 'default':
            return
        # Your migration code goes here

    class Migration(migrations.Migration):

        dependencies = [
            # Dependencies to other migrations
        ]

        operations = [
            migrations.RunPython(forwards),
        ]

.. versionadded:: 1.8

You can also provide hints that will be passed to the :meth:`allow_migrate()`
method of database routers as ``**hints``:

.. snippet::
    :filename: myapp/dbrouters.py

    class MyRouter(object):

        def allow_migrate(self, db, app_label, model_name=None, **hints):
            if 'target_db' in hints:
                return db == hints['target_db']
            return True

Then, to leverage this in your migrations, do the following::

    from django.db import migrations

    def forwards(apps, schema_editor):
        # Your migration code goes here
        ...

    class Migration(migrations.Migration):

        dependencies = [
            # Dependencies to other migrations
        ]

        operations = [
            migrations.RunPython(forwards, hints={'target_db': 'default'}),
        ]

If your ``RunPython`` or ``RunSQL`` operation only affects one model, it's good
practice to pass ``model_name`` as a hint to make it as transparent as possible
to the router. This is especially important for reusable and third-party apps.

Migrations that add unique fields
=================================

Applying a "plain" migration that adds a unique non-nullable field to a table
with existing rows will raise an error because the value used to populate
existing rows is generated only once, thus breaking the unique constraint.

Therefore, the following steps should be taken. In this example, we'll add a
non-nullable :class:`~django.db.models.UUIDField` with a default value. Modify
the respective field according to your needs.

* Add the field on your model with ``default=uuid.uuid4`` and ``unique=True``
  arguments (choose an appropriate default for the type of the field you're
  adding).

* Run the :djadmin:`makemigrations` command. This should generate a migration
  with an ``AddField`` operation.

* Generate two empty migration files for the same app by running
  ``makemigrations myapp --empty`` twice. We've renamed the migration files to
  give them meaningful names in the examples below.

* Copy the ``AddField`` operation from the auto-generated migration (the first
  of the three new files) to the last migration and change ``AddField`` to
  ``AlterField``. For example:

  .. snippet::
    :filename: 0006_remove_uuid_null.py

    # -*- coding: utf-8 -*-
    # Generated by Django A.B on YYYY-MM-DD HH:MM
    from __future__ import unicode_literals

    from django.db import migrations, models
    import uuid


    class Migration(migrations.Migration):

        dependencies = [
            ('myapp', '0005_populate_uuid_values'),
        ]

        operations = [
            migrations.AlterField(
                model_name='mymodel',
                name='uuid',
                field=models.UUIDField(default=uuid.uuid4, unique=True),
            ),
        ]

* Edit the first migration file. The generated migration class should look
  similar to this:

  .. snippet::
    :filename: 0004_add_uuid_field.py

    class Migration(migrations.Migration):

        dependencies = [
            ('myapp', '0003_auto_20150129_1705'),
        ]

        operations = [
            migrations.AddField(
                model_name='mymodel',
                name='uuid',
                field=models.UUIDField(default=uuid.uuid4, unique=True),
            ),
        ]

  Change ``unique=True`` to ``null=True`` -- this will create the intermediary
  null field and defer creating the unique constraint until we've populated
  unique values on all the rows.

* In the first empty migration file, add a
  :class:`~django.db.migrations.operations.RunPython` or
  :class:`~django.db.migrations.operations.RunSQL` operation to generate a
  unique value (UUID in the example) for each existing row. For example:

  .. snippet::
    :filename: 0005_populate_uuid_values.py

    # -*- coding: utf-8 -*-
    # Generated by Django A.B on YYYY-MM-DD HH:MM
    from __future__ import unicode_literals

    from django.db import migrations, models
    import uuid

    def gen_uuid(apps, schema_editor):
        MyModel = apps.get_model('myapp', 'MyModel')
        for row in MyModel.objects.all():
            row.uuid = uuid.uuid4()
            row.save()

    class Migration(migrations.Migration):

        dependencies = [
            ('myapp', '0004_add_uuid_field'),
        ]

        operations = [
            # omit reverse_code=... if you don't want the migration to be reversible.
            migrations.RunPython(gen_uuid, reverse_code=migrations.RunPython.noop),
        ]

* Now you can apply the migrations as usual with the :djadmin:`migrate` command.

  Note there is a race condition if you allow objects to be created while this
  migration is running. Objects created after the ``AddField`` and before
  ``RunPython`` will have their original ``uuid``’s overwritten.

Controlling the order of migrations
===================================

Django determines the order in which migrations should be applied not by the
filename of each migration, but by building a graph using two properties on the
``Migration`` class: ``dependencies`` and ``run_before``.

If you've used the :djadmin:`makemigrations` command you've probably
already seen ``dependencies`` in action because auto-created
migrations have this defined as part of their creation process.

The ``dependencies`` property is declared like this::

    from django.db import migrations

    class Migration(migrations.Migration):

        dependencies = [
            ('myapp', '0123_the_previous_migration'),
        ]

Usually this will be enough, but from time to time you may need to
ensure that your migration runs *before* other migrations. This is
useful, for example, to make third-party apps' migrations run *after*
your :setting:`AUTH_USER_MODEL` replacement.

To achieve this, place all migrations that should depend on yours in
the ``run_before`` attribute on your ``Migration`` class::

    class Migration(migrations.Migration):
        ...

        run_before = [
            ('third_party_app', '0001_do_awesome'),
        ]

Prefer using ``dependencies`` over ``run_before`` when possible. You should
only use ``run_before`` if it is undesirable or impractical to specify
``dependencies`` in the migration which you want to run after the one you are
writing.
