diff --git a/docs/topics/db/multi-db.txt b/docs/topics/db/multi-db.txt index 27dc2261ca..e3f62ea71b 100644 --- a/docs/topics/db/multi-db.txt +++ b/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 ` + 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.