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Fixed #13316 -- Added clarifying note about cross-database relations. 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13178 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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Russell Keith-Magee 2010-05-09 07:44:06 +00:00
parent ced4dd2aad
commit 9320c866ff
1 changed files with 34 additions and 7 deletions
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41
docs/topics/db/multi-db.txt
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@ -193,13 +193,11 @@ An example
intentionally ignores some complex issues in order to
demonstrate how routers are used.
This example won't work on Postgres, Oracle, or MySQL with InnoDB
tables if any of the models in ``myapp`` contain foreign keys to
models outside of the ``other`` database. ForeignKeys to a remote
database introduce referential integrity problems that Django can't
currently handle. However, if you're using SQLite or MySQL with MyISAM
tables, there is no referential integrity checking, so you will be
able to define cross-database foreign keys.
This example won't work if any of the models in ``myapp`` contain
relationships to models outside of the ``other`` database.
:ref:`Cross-database relationships <no_cross_database_relations>`
introduce referential integrity problems that Django can't
currently handle.
The master/slave configuration described is also flawed -- it
doesn't provide any solution for handling replication lag (i.e.,
@ -547,3 +545,32 @@ alias::
from django.db import connections
cursor = connections['my_db_alias'].cursor()
Limitations of multiple databases
=================================
.. _no_cross_database_relations:
Cross-database relations
------------------------
Django doesn't currently provide any support for foreign key or
many-to-many relationships spanning multiple databases. If you
have used a router to partition models to different databases,
any foreign key and many-to-many relationships defined by those
models must be internal to a single database.
This is because of referential integrity. In order to maintain a
relationship between two objects, Django needs to know that the
primary key of the related object is valid. If the primary key is
stored on a separate database, it's not possible to easily evaluate
the validity of a primary key.
If you're using Postgres, Oracle, or MySQL with InnoDB, this is
enforced at the database integrity level -- database level key
constraints prevent the creation of relations that can't be validated.
However, if you're using SQLite or MySQL with MyISAM tables, there is
no enforced referential integrity; as a result, you may be able to
'fake' cross database foreign keys. However, this configuration is not
officially supported by Django.