diff --git a/docs/howto/writing-migrations.txt b/docs/howto/writing-migrations.txt index 2daf6ade7a5..1e77434b956 100644 --- a/docs/howto/writing-migrations.txt +++ b/docs/howto/writing-migrations.txt @@ -183,3 +183,44 @@ the respective field according to your needs. 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 ``dependecies`` 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.