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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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commit 9320c866ffbfba247641f42db9954c50ad0a5c6e 1 parent ced4dd2
@freakboy3742 freakboy3742 authored
Showing with 34 additions and 7 deletions.
  1. +34 −7 docs/topics/db/multi-db.txt
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41 docs/topics/db/multi-db.txt
@@ -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.
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