Small start to migrations documentation

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Andrew Godwin 2013-07-25 14:45:38 +01:00
parent 00276e0414
commit 06103c8ef5
4 changed files with 164 additions and 39 deletions

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@ -71,6 +71,9 @@ manipulating the data of your Web application. Learn more about it below:
:doc:`Instance methods <ref/models/instances>` |
:doc:`Accessing related objects <ref/models/relations>`
* **Migrations:**
:doc:`Introduction to Migrations<topics/migrations>`
* **Advanced:**
:doc:`Managers <topics/db/managers>` |
:doc:`Raw SQL <topics/db/sql>` |

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@ -572,6 +572,48 @@ Use the ``--keep-pot`` option to prevent django from deleting the temporary
.pot file it generates before creating the .po file. This is useful for
debugging errors which may prevent the final language files from being created.
makemigrations [<appname>]
--------------------------
.. django-admin:: makemigrations
Creates new migrations based on the changes detected to your models.
Migrations, their relationship with apps and more are covered in depth in
:doc:`the migrations documentation</topics/migrations>`.
Providing one or more app names as arguments will limit the migrations created
to the app specified and any dependencies needed (the table at the other end
of a ForeignKey, for example)
.. django-admin-option:: --empty
The ``--empty`` option will cause ``makemigrations`` to output an empty
migration for the specified apps, for manual editing. This option is only
for advanced users and should not be used unless you are familiar with
the migration format, migration operations and the dependencies between
your migrations.
migrate [<appname> [<migrationname>]]
-------------------------------------
.. django-admin:: migrate
Synchronises the database state with the current set of models and migrations.
Migrations, their relationship with apps and more are covered in depth in
:doc:`the migrations documentation</topics/migrations>`.
The behaviour of this command changes depending on the arguments provided:
* No arguments: All migrated apps have all of their migrations run,
and all unmigrated apps are synchronized with the database,
* ``<appname>``: The specified app has its migrations run, up to the most
recent migration. This may involve running other apps' migrations too, due
to dependencies.
* ``<appname> <migrationname>``: Brings the database schema to a state where it
would have just run the given migration, but no further - this may involve
unapplying migrations if you have previously migrated past the named
migration. Use the name `zero` to unapply all migrations for an app.
runfcgi [options]
-----------------
@ -1107,47 +1149,13 @@ syncdb
.. django-admin:: syncdb
Creates the database tables for all apps in :setting:`INSTALLED_APPS` whose
tables have not already been created.
.. deprecated:: 1.7
Use this command when you've added new applications to your project and want to
install them in the database. This includes any apps shipped with Django that
might be in :setting:`INSTALLED_APPS` by default. When you start a new project,
run this command to install the default apps.
This command has been deprecated in favour of the :djadmin:`migrate`
command, which performs both the old behaviour as well as executing
migrations. It is now just an alias to that command.
.. admonition:: Syncdb will not alter existing tables
``syncdb`` will only create tables for models which have not yet been
installed. It will *never* issue ``ALTER TABLE`` statements to match
changes made to a model class after installation. Changes to model classes
and database schemas often involve some form of ambiguity and, in those
cases, Django would have to guess at the correct changes to make. There is
a risk that critical data would be lost in the process.
If you have made changes to a model and wish to alter the database tables
to match, use the ``sql`` command to display the new SQL structure and
compare that to your existing table schema to work out the changes.
If you're installing the ``django.contrib.auth`` application, ``syncdb`` will
give you the option of creating a superuser immediately.
``syncdb`` will also search for and install any fixture named ``initial_data``
with an appropriate extension (e.g. ``json`` or ``xml``). See the
documentation for ``loaddata`` for details on the specification of fixture
data files.
The :djadminopt:`--noinput` option may be provided to suppress all user
prompts.
The :djadminopt:`--database` option can be used to specify the database to
synchronize.
--no-initial-data
~~~~~~~~~~~~~~~~~
.. versionadded:: 1.5
Use ``--no-initial-data`` to avoid loading the initial_data fixture.
Alias for :djadmin:`migrate`.
test <app or test identifier>
-----------------------------

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@ -12,6 +12,7 @@ Introductions to all the key parts of Django you'll need to know:
forms/index
templates
class-based-views/index
migrations
files
testing/index
auth/index

113
docs/topics/migrations.txt Normal file
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@ -0,0 +1,113 @@
==========
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.