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[1.10.x] Refs #25850, #27142, #27110 -- Documented migration history consistency checks.

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Backport of c93ac9cf42 from master
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Shai Berger
2016-09-01 23:19:47 +03:00
committed by Tim Graham
parent 02c6122938
commit 57f7d8968d
3 changed files with 39 additions and 5 deletions
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17
docs/topics/db/multi-db.txt
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@@ -98,10 +98,22 @@ the database name would raise an error. For the second example::
Using other management commands
-------------------------------
The other ``django-admin`` commands that interact with the database operate in
Most other ``django-admin`` commands that interact with the database operate in
the same way as :djadmin:`migrate` -- they only ever operate on one database at
a time, using ``--database`` to control the database used.
An exception to this rule is the :djadmin:`makemigrations` command. It
validates the migration history in the databases to catch problems with the
existing migration files (which could be caused by editing them) before
creating new migrations. By default, it checks only the ``default`` database,
but it consults the the :meth:`allow_migrate` method of :ref:`routers
<topics-db-multi-db-routing>` if any are installed.
.. versionchanged:: 1.10
Migration consistency checks were added. Checks based on database routers
were added in 1.10.1.
.. _topics-db-multi-db-routing:
Automatic database routing
@@ -188,7 +200,8 @@ A database Router is a class that provides up to four methods:
``model_name`` will be silently skipped when running :djadmin:`migrate` on
the ``db``. Changing the behavior of ``allow_migrate()`` for models that
already have migrations may result in broken foreign keys, extra tables,
or missing tables.
or missing tables. When :djadmin:`makemigrations` verifies the migration
history, it skips databases where no app is allowed to migrate.
A router doesn't have to provide *all* these methods -- it may omit one
or more of them. If one of the methods is omitted, Django will skip

20
docs/topics/migrations.txt
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@@ -300,6 +300,26 @@ Django checks that all of the respective columns already exist in the database
and fake-applies the migration if so. Without ``--fake-initial``, initial
migrations are treated no differently from any other migration.
.. _migration-history-consistency:
History consistency
-------------------
As previously discussed, you may need to linearize migrations manually when two
development branches are joined. While editing migration dependencies, you can
inadvertently create an inconsistent history state where a migration has been
applied but some of its dependencies haven't. This is a strong indication that
the dependencies are incorrect, so Django will refuse to run migrations or make
new migrations until it's fixed. When using multiple databases, you can use the
:meth:`allow_migrate` method of :ref:`database routers
<topics-db-multi-db-routing>` to control which databases
:djadmin:`makemigrations` checks for consistent history.
.. versionchanged:: 1.10
Migration consistency checks were added. Checks based on database routers
were added in 1.10.1.
Adding migrations to apps
=========================