django/docs/howto/writing-migrations.txt

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2015-02-04 02:09:54 +08:00
===========================
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, model, **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'}),
]