Fixed #22620 -- Emphasized role of DATABASE_ROTERS in multi-db docs.

of your project, you need to be careful to always specify the database

that you want to use. Django requires that a ``default`` database entry

be defined, but the parameters dictionary can be left blank if it will not be

used. The following is an example ``settings.py`` snippet defining two

non-default databases, with the ``default`` entry intentionally left empty::

used. You must setup :setting:`DATABASE_ROUTERS` for all of your apps' models,

including those in any contrib and third-party apps you are using, so that no

queries are routed to the default database in order to do this. The following

is an example ``settings.py`` snippet defining two non-default databases, with

the ``default`` entry intentionally left empty::

    DATABASES = {

        'default': {},

router<topics-db-multi-db-routing>` that implements a policy

constraining the availability of particular models.

Alternatively, if you want fine-grained control of synchronization,

you can pipe all or part of the output of :djadmin:`sqlall` for a

particular application directly into your database prompt, like this::

    $ ./manage.py sqlall sales | ./manage.py dbshell

Using other management commands

-------------------------------

For common setups with multiple databases, it isn't useful to have these

objects in more than one database. Common setups include primary/replica and

connecting to external databases. Therefore, it's recommended:

- either to run :djadmin:`migrate` only for the default database;

- or to write :ref:`database router<topics-db-multi-db-routing>` that allows

  synchronizing these three models only to one database.

connecting to external databases. Therefore, it's recommended to write a

:ref:`database router<topics-db-multi-db-routing>` that allows synchronizing

these three models to only one database. Use the same approach for contrib

and third-party apps that don't need their tables in multiple databases.

.. warning::