Author: russellm
Date: 2010-05-09 02:44:06 -0500 (Sun, 09 May 2010)
New Revision: 13178
Modified:
django/trunk/docs/topics/db/multi-db.txt
Log:
Fixed #13316 -- Added clarifying note about cross-database relations.
Modified: django/trunk/docs/topics/db/multi-db.txt
===================================================================
--- django/trunk/docs/topics/db/multi-db.txt 2010-05-09 06:51:07 UTC (rev
13177)
+++ django/trunk/docs/topics/db/multi-db.txt 2010-05-09 07:44:06 UTC (rev
13178)
@@ -193,13 +193,11 @@
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 @@
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.
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