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

    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.,

    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.