django/docs/topics/db/multi-db.txt

.. _topics-db-multi-db:

==================
Multiple Databases
==================

.. versionadded:: 1.2

This topic guide describes Django's support for interacting with multiple
databases.  Most of the rest of Django's documentation assumes you are
interacting with a single database.  While none of this documentation is
incorrect, to fully interact with multiple databases additional steps must be
taken.

Defining your databases
=======================

The first step to using more than one database with Django is to tell Django
about the database servers you'll be using.  This is done using the
:setting:`DATABASES` setting.  This setting maps database aliases, which are
a way to refer to a specific database throughout Django, to a dictionary of
settings for that specific connection.  The settings in the inner dictionaries
are described fully in the :setting:`DATABASES` documentation.  The important
thing to note is that your primary database should have the alias
``'default'``, and any additional databases you have can have whatever alias
you choose.  If at any time you attempt to access a database that isn't defined
in your :setting:`DATABASES` setting then Django will raise a
``django.db.utils.ConnectionDoesNotExist`` exception.

Selecting a database for a ``QuerySet``
=======================================

It is possible to select the database for a ``QuerySet`` at any point during
it's construction.  To choose the database that a query will be preformed
against simply call the ``using()`` method on the ``QuerySet`` with the sole
argument being the database alias.

Select a database to save to
============================

To choose what database to save a model to, provide a ``using`` keyword
argument to ``Model.save()``.  For example if you had a user model that you
wanted to save to the ``'legacy_users'`` database you would do::

    >>> user_obj.save(using='legacy_users')

To save the user.

Moving an object from one database to another
---------------------------------------------

If you have saved an instance to one database, it might be tempting to use
``save(using=...)`` as a way to migrate the instance to a new database. However,
if you don't take appropriate steps, this could have some unexpected consequences.

Consider the following example::

    >>> p = Person(name='Fred')
    >>> p.save(using='first') # (1)
    # some other processing ...
    >>> p.save(using='second') # (2)

In statement 1, a new Person object is saved to the ``first``
database. At this time, ``p`` doesn't have a primary key, so Django
issues a SQL ``INSERT`` statement. This creates a primary key, and
Django assigns that primary key to ``p``.

When the save occurs in statement 2, ``p`` already has a primary key
value, and Django will attempt to use that primary key on the new
database. If the primary key value isn't in use in the ``second``
database, then you won't have any problems -- the object will be
copied to the new databse.

However, if the primary key of ``p`` is already in use on the
``second`` database, the existing object on the ``second`` database
will be lost when ``p`` is saved.

There are two ways to avoid this outcome. Firstly, you can clear the
primary key of the instance. If an object has no primary key, Django
will treat it as a new object, avoiding any loss of data on the
``second`` database::

    >>> p = Person(name='Fred')
    >>> p.save(using='first')
    # some other processing ...
    >>> p.pk = None # Clear the PK
    >>> p.save(using='second') # Write a completely new object

Secondly, you can use the ``force_insert`` option to ``save()`` to ensure that
Django does a SQL ``INSERT``::

    >>> p = Person(name='Fred')
    >>> p.save(using='first')
    # some other processing ...
    >>> p.save(using='second', force_insert=True)

This will ensure that the person named ``Fred`` will have the same
primary key on both databases. If that primary key is already in use
when you try to save onto the ``second`` database, an error will be
raised.

Select a database to delete from
================================

By default, a call to delete an existing object will be executed on the
same database that was used to retrieve the object in the first place::

    >>> user_obj = User.objects.using('legacy_users').get(username='fred')
    >>> user_obj.delete() # will delete from the `legacy_users` database

If you want to specify the database from which a model will be
deleted, you can use a ``using`` keyword argument to the
``Model.delete()`` method. This argument is analogous to the ``using``
keyword argument to ``save()``. For example if you were migrating a
user from the ``'legacy_users'`` database to the ``'new_users'``
database you might use the commands::

    >>> user_obj.save(using='new_users')
    >>> user_obj.delete(using='legacy_users')

Using ``Managers`` with multiple databases
==========================================

When you call ``using()`` Django returns a ``QuerySet`` that will be evaluated
against that database.  However, sometimes you want to chain ``using()``
together with a cusotm manager method that doesn't return a ``QuerySet``,
such as the ``get_by_natural_key`` method.  To solve this issue you can use the
``db_manager()`` method on a manager.  This method returns a copy of the
*manager* bound to that specific database.  This let's you do things like::

    >>> Book.objects.db("other").get_by_natural_key(...)

If you are overriding ``get_query_set()`` on your manager you must be sure to
either, a) call the method on the parent (using ``super()``), or b) do the
appropriate handling of the ``_db`` attribute on the manager.  For example if
you wanted to return a custom ``QuerySet`` class from the ``get_query_set``
method you could do this::

    class MyManager(models.Manager):
        ...
        def get_query_set(self):
            qs = CustomQuerySet(self.model)
            if self._db is not None:
                qs = qs.using(self._db)
            return qs

Exposing multiple databases in Django's admin interface
=======================================================

Django's admin doesn't have any explicit support for multi databases.
If you want to provide an admin interface for a model on a database
other than ``default``, you need to write custom
:class:`~django.contrib.admin.ModelAdmin` classes that will direct the
admin to use a specific database for content.

There are four methods that require customization on a ModelAdmin
object::

    class MultiDBModelAdmin(admin.ModelAdmin):
        # A handy constant for the name of the alternate database
        using = 'other'

        def save_model(self, request, obj, form, change):
            # Tell Django to save objects to the 'other' database
            obj.save(using=self.using)

        def queryset(self, request):
            # Tell Django to look for objects on the 'other' database
            return super(MultiDBModelAdmin, self).queryset(request).using(self.using)

        def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
            # Tell Django to populate ForeignKey widgets using a query
            # on the 'other' database
            return super(MultiDBModelAdmin, self).formfield_for_foreignkey(db_field, request=request, using=self.using, **kwargs)

        def formfield_for_manytomany(self, db_field, request=None, **kwargs):
            # Tell Django to populate ManyToMany widgets using a query
            # on the 'other' database
            return super(MultiDBModelAdmin, self).formfield_for_manytomany(db_field, request=request, using=self.using, **kwargs)

The implementation provided here implements a multi-db strategy where
all objects of a given type are stored on a specific database (e.g.,
all ``User`` objects are on the ``other`` database). If your usage of
multi-db is more complex, your ModelAdmin will need to reflect that
strategy.

Inlines can be handled in a similar fashion -- they require just three
customized methods::

    class MultiDBTabularInline(admin.TabularInline):
        using = 'other'

        def queryset(self, request):
            # Tell Django to look for inline objects on the 'other' database
            return super(MultiDBTabularInline, self).queryset(request).using(self.using)

        def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
            # Tell Django to populate ForeignKey widgets using a query
            # on the 'other' database
            return super(MultiDBTabularInline, self).formfield_for_foreignkey(db_field, request=request, using=self.using, **kwargs)

        def formfield_for_manytomany(self, db_field, request=None, **kwargs):
            # Tell Django to populate ManyToMany widgets using a query
            # on the 'other' database
            return super(MultiDBTabularInline, self).formfield_for_manytomany(db_field, request=request, using=self.using, **kwargs)

Once you have written your model admin definitions, they can be
registered with any Admin instance::

    from django.contrib import admin

    # Specialize the multi-db admin objects for use with specific models
    class BookInline(MultiDBTabularInline):
        model = Book

    class PublisherAdmin(MultiDBModelAdmin):
        inlines = [BookInline]

        admin.site.register

    admin.site.register(Author, MultiDBModelAdmin)
    admin.site.register(Publisher, PublisherAdmin)

    othersite = admin.Site('othersite')
    othersite.register(Publisher, MultiDBModelAdmin)

This example sets up two admin sites. On the first site, the
``Author`` and ``Publisher`` objects are exposed; ``Publisher``
objects have an tabular inline showing books published by that
publisher. The second site exposes just publishers, without the
inlines.