==========
Migrations
==========

.. module:: django.db.migrations
   :synopsis: Schema migration support for Django models

.. versionadded:: 1.7

Migrations are Django's way of propagating changes you make to your models
(adding a field, deleting a model, etc.) into your database schema. They're
designed to be mostly automatic, but you'll need to know when to make
migrations, when to run them, and the common problems you might run into.

A Brief History
---------------

Prior to version 1.7, Django only supported adding new models to the
database; it was not possible to alter or remove existing models via the
``syncdb`` command (the predecessor to ``migrate``).

Third-party tools, most notably `South <http://south.aeracode.org>`_,
provided support for these additional types of change, but it was considered
important enough that support was brought into core Django.

Two Commands
------------

There are two commands which you will use to interact with migrations
and Django's handling of database schema:

* :djadmin:`migrate`, which is responsible for applying migrations, as well as
  unapplying and listing their status.

* :djadmin:`makemigrations`, which is responsible for creating new migrations
  based on the changes you have made to your models.

It's worth noting that migrations are created and run on a per-app basis.
In particular, it's possible to have apps that *do not use migrations* (these
are referred to as "unmigrated" apps) - these apps will instead mimic the
legacy behaviour of just adding new models.

You should think of migrations as a version control system for your database
schema. ``makemigrations`` is responsible for packaging up your model changes
into individual migration files - analagous to commits - and ``migrate`` is
responsible for applying those to your database.

The migration files for each app live in a "migrations" directory inside
of that app, and are designed to be committed to, and distributed as part
of, its codebase. You should be making them once on your development machine
and then running the same migrations on your colleagues' machines, your
staging machines and eventually your production machines.

Migrations will run the same way every time and produce consistent results,
meaning that what you see in development and staging is exactly what will
happen in production - no unexpected surprises.

Backend Support
---------------

Migrations are supported on all backends that Django ships with, as well
as any third-party backends if they have programmed in support for schema
alteration (done via the SchemaEditor class).

However, some databases are more capable than others when it comes to
schema migrations; some of the caveats are covered below.

PostgreSQL
~~~~~~~~~~

PostgreSQL is the most capable of all the databases here in terms of schema
support; the only caveat is that adding columns with default values will
lock a table for a time proportional to the number of rows in it.

For this reason, it's recommended you always create new columns with
``null=True``, as this way they will be added immediately.

MySQL
~~~~~

MySQL lacks support for transactions around schema alteration operations,
meaning that if a migration fails to apply you will have to manually unpick
the changes in order to try again (it's impossible to roll back to an
earlier point).

In addition, MySQL will lock tables for almost every schema operation and
generally takes a time proportional to the number of rows in the table to
add or remove columns. On slower hardware this can be worse than a minute
per million rows - adding a few columns to a table with just a few million
rows could lock your site up for over ten minutes.

Finally, MySQL has reasonably small limits on name lengths for columns, tables
and indexes, as well as a limit on the combined size of all columns an index
covers. This means that indexes that are possible on other backends will
fail to be created under MySQL.

SQLite
~~~~~~

SQLite has very little built-in schema alteration support, and so Django
attempts to emulate it by:

* Creating a new table with the new schema
* Copying the data across
* Dropping the old table
* Renaming the new table to match the original name

This process generally works well, but it can be slow and occasionally
buggy. It is not recommended that you run and migrate SQLite in a
production environment unless you are very aware of the risks and
its limitations; the support Django ships with is designed to allow
developers to use SQLite on their local machines to develop less complex
Django projects without the need for a full database.

Workflow
--------

Working with migrations is simple. Make changes to your models - say, add
a field and remove a model - and then run :djadmin:`makemigrations`::

    $ python manage.py makemigrations
    Migrations for 'books':
      0003_auto.py:
        - Alter field author on book

Your models will be scanned and compared to the versions currently
contained in your migration files, and then a new set of migrations
will be written out. Make sure to read the output to see what
``makemigrations`` thinks you have changed - it's not perfect, and for
complex changes it might not be detecting what you expect.

Once you have your new migration files, you should apply them to your
database to make sure they work as expected::

    $ python manage.py migrate
    Operations to perform:
      Synchronize unmigrated apps: sessions, admin, messages, auth, staticfiles, contenttypes
      Apply all migrations: books
    Synchronizing apps without migrations:
      Creating tables...
      Installing custom SQL...
      Installing indexes...
    Installed 0 object(s) from 0 fixture(s)
    Running migrations:
      Applying books.0003_auto... OK

The command runs in two stages; first, it synchronizes unmigrated apps
(performing the same functionality that ``syncdb`` used to provide), and
then it runs any migrations that have not yet been applied.

Once the migration is applied, commit the migration and the models change
to your version control system as a single commit - that way, when other
developers (or your production servers) check out the code, they'll
get both the changes to your models and the accompanying migration at the
same time.

Version control
~~~~~~~~~~~~~~~

Because migrations are stored in version control, you'll occasionally
come across situations where you and another developer have both committed
a migration to the same app at the same time, resulting in two migrations
with the same number.

Don't worry - the numbers are just there for developers' reference, Django
just cares that each migration has a different name. Migrations specify which
other migrations they depend on - including earlier migrations in the same
app - in the file, so it's possible to detect when there's two new migrations
for the same app that aren't ordered.

When this happens, Django will prompt you and give you some options. If it
thinks it's safe enough, it will offer to automatically linearise the two
migrations for you. If not, you'll have to go in and modify the migrations
yourself - don't worry, this isn't difficult, and is explained more in
:ref:`migration-files` below.

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
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
throw an error.

This dependency behaviour affects most migration operations where you
restrict to a single app. Restricting to a single app (either in
``makemigrations`` or ``migrate``) is a best-efforts promise, and not
a guarantee; any other apps that need to be used to get dependencies correct
will be.

.. migration-files:

Migration files
---------------

Migrations are stored as an on-disk format, referred to here as
"migration files". These files are actually just normal Python files with
an agreed-upon object layout, written in a declarative style.

A basic migration file looks like this::

    from django.db import migrations, models

    class Migration(migrations.Migration):

        dependencies = [("migrations", "0001_initial")]

        operations = [
            migrations.DeleteModel("Tribble"),
            migrations.AddField("Author", "rating", models.IntegerField(default=0)),
        ]

What Django looks for when it loads a migration file (as a Python module) is
a subclass of ``django.db.migrations.Migration`` called ``Migration``. It then
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.

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
builds an in-memory representation of all of the schema changes to all apps,
and uses this to generate the SQL which makes the schema changes.

That in-memory structure is also used to work out what the differences are
between your models and the current state of your migrations; Django runs
through all the changes, in order, on an in-memory set of models to come
up with the state of your models last time you ran ``makemigrations``. It
then uses these models to compare against the ones in your ``models.py`` files
to work out what you have changed.

You should rarely, if ever, need to edit migration files by hand, but
it's entirely possible to write them manually if you need to. Some of the
more complex operations are not autodetectable and are only available via
a hand-written migration, so don't be scared about editing them if you have to.

Adding migrations to apps
-------------------------

Adding migrations to new apps is straightforward - they come preconfigured to
accept migrations, and so just run :djadmin:`makemigrations` once you've made
some changes.

If your app already has models and database tables, and doesn't have migrations
yet (for example, you created it against a previous Django version), you'll
need to convert it to use migrations; this is a simple process::

    python manage.py makemigrations --force yourappname

This will make a new initial migration for your app (the ``--force`` argument
is to override Django's default behaviour, as it thinks your app does not want
migrations). Now, when you run :djadmin:`migrate`, Django will detect that
you have an initial migration *and* that the tables it wants to create already
exist, and will mark the migration as already applied.

Note that this only works given two things:

* You have not changed your models since you made their tables. For migrations
  to work, you must make the initial migration *first* and then make changes,
  as Django compares changes against migration files, not the database.

* You have not manually edited your database - Django won't be able to detect
  that your database doesn't match your models, you'll just get errors when
  migrations try and modify those tables.
