310 lines
12 KiB
Plaintext
310 lines
12 KiB
Plaintext
.. _topics-db-multi-db:
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==================
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Multiple databases
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==================
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.. versionadded:: 1.2
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This topic guide describes Django's support for interacting with multiple
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databases. Most of the rest of Django's documentation assumes you are
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interacting with a single database. If you want to interact with multiple
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databases, you'll need to take some additional steps.
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Defining your databases
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=======================
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The first step to using more than one database with Django is to tell
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Django about the database servers you'll be using. This is done using
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the :setting:`DATABASES` setting. This setting maps database aliases,
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which are a way to refer to a specific database throughout Django, to
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a dictionary of settings for that specific connection. The settings in
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the inner dictionaries are described fully in the :setting:`DATABASES`
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documentation.
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Regardless of how many databases you have, you *must* have a database
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named ``'default'``. Any additional databases can have whatever alias
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you choose.
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The following is an example ``settings.py`` snippet defining two
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databases -- a default PostgreSQL database and a MySQL database called
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``users``:
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.. code-block:: python
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DATABASES = {
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'default': {
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'NAME': 'app_data',
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'ENGINE': 'django.db.backends.postgresql_psycopg2',
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'USER': 'postgres_user',
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'PASSWORD': 's3krit'
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},
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'users': {
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'NAME': 'user_data'
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'ENGINE': 'django.db.backends.mysql',
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'USER': 'mysql_user',
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'PASSWORD': 'priv4te'
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}
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}
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If you attempt to access a database that you haven't defined in your
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:setting:`DATABASES` setting, Django will raise a
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``django.db.utils.ConnectionDoesNotExist`` exception.
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Synchronizing your databases
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============================
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The :djadmin:`syncdb` management command operates on one database at a
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time. By default, it operates on the ``default`` database, but by
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providing a :djadminopt:`--database` argument, you can tell syncdb to
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synchronize a different database. So, to synchronize all models onto
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all databases in our example, you would need to call::
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$ ./manage.py syncdb
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$ ./manage.py syncdb --database=users
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If you don't want every application to be synchronized onto a
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particular database. you can specify the :djadminopt:`--exclude`
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argument to :djadmin:`syncdb`. The :djadminopt:`--exclude` option
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lets you prevent a specific application or applications from
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being synchronized. For example, if you don't want the ``sales``
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application to be in the ``users`` database, you could run::
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$ ./manage.py syncdb --database=users --exclude=sales
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Alternatively, if you want fine-grained control of synchronization,
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you can pipe all or part of the output of :djadmin:`sqlall` for a
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particular application directly into your database prompt, like this::
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$ ./manage.py sqlall sales | ./manage.py dbshell
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Using other management commands
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-------------------------------
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The other ``django-admin.py`` commands that interact with the database
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operate in the same way as :djadmin:`syncdb` -- they only ever operate
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on one database at a time, using :djadminopt:`--database` to control
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the database used.
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Selecting a database for a ``QuerySet``
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=======================================
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You can select the database for a ``QuerySet`` at any point in the ``QuerySet``
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"chain." Just call ``using()`` on the ``QuerySet`` to get another ``QuerySet``
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that uses the specified database.
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``using()`` takes a single argument: the alias of the database on which you
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want to run the query. For example:
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.. code-block:: python
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# This will run on the 'default' database.
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>>> Author.objects.all()
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# So will this.
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>>> Author.objects.using('default').all()
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# This will run on the 'other' database.
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>>> Author.objects.using('other').all()
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Selecting a database for ``save()``
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===================================
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Use the ``using`` keyword to ``Model.save()`` to specify to which database the
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data should be saved.
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For example, to save an object to the ``legacy_users`` database, you'd use this::
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>>> my_object.save(using='legacy_users')
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If you don't specify ``using``, the ``save()`` method will always save into the
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default database.
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Moving an object from one database to another
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---------------------------------------------
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If you've saved an instance to one database, it might be tempting to use
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``save(using=...)`` as a way to migrate the instance to a new database. However,
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if you don't take appropriate steps, this could have some unexpected consequences.
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Consider the following example::
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>>> p = Person(name='Fred')
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>>> p.save(using='first') # (statement 1)
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>>> p.save(using='second') # (statement 2)
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In statement 1, a new ``Person`` object is saved to the ``first``
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database. At this time, ``p`` doesn't have a primary key, so Django
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issues a SQL ``INSERT`` statement. This creates a primary key, and
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Django assigns that primary key to ``p``.
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When the save occurs in statement 2, ``p`` already has a primary key
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value, and Django will attempt to use that primary key on the new
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database. If the primary key value isn't in use in the ``second``
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database, then you won't have any problems -- the object will be
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copied to the new database.
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However, if the primary key of ``p`` is already in use on the
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``second`` database, the existing object in the ``second`` database
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will be overridden when ``p`` is saved.
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You can avoid this in two ways. First, you can clear the primary key
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of the instance. If an object has no primary key, Django will treat it as
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a new object, avoiding any loss of data on the ``second`` database::
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>>> p = Person(name='Fred')
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>>> p.save(using='first')
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>>> p.pk = None # Clear the primary key.
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>>> p.save(using='second') # Write a completely new object.
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The second option is to use the ``force_insert`` option to ``save()`` to ensure
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that Django does a SQL ``INSERT``::
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>>> p = Person(name='Fred')
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>>> p.save(using='first')
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>>> p.save(using='second', force_insert=True)
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This will ensure that the person named ``Fred`` will have the same
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primary key on both databases. If that primary key is already in use
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when you try to save onto the ``second`` database, an error will be
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raised.
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Selecting a database to delete from
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===================================
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By default, a call to delete an existing object will be executed on the
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same database that was used to retrieve the object in the first place::
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>>> u = User.objects.using('legacy_users').get(username='fred')
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>>> u.delete() # will delete from the `legacy_users` database
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To specify the database from which a model will be deleted, pass a
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``using`` keyword argument to the ``Model.delete()`` method. This argument
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works just like the ``using`` keyword argument to ``save()``.
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For example, if you're migrating a user from the ``legacy_users`` database
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to the ``new_users`` database, you might use these commands::
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>>> user_obj.save(using='new_users')
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>>> user_obj.delete(using='legacy_users')
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Using managers with multiple databases
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======================================
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Use the ``db_manager()`` method on managers to give managers access to a
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non-default database.
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For example, say you have a custom manager method that touches the database --
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``User.objects.create_user()``. Because ``create_user()`` is a
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manager method, not a ``QuerySet`` method, you can't do
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``User.objects.using('new_users').create_user()``. (The ``create_user()`` method
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is only available on ``User.objects``, the manager, not on ``QuerySet`` objects
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derived from the manager.) The solution is to use ``db_manager()``, like this::
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User.objects.db_manager('new_users').create_user(...)
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``db_manager()`` returns a copy of the manager bound to the database you specify.
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Using ``get_query_set()`` with multiple databases
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-------------------------------------------------
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If you're overriding ``get_query_set()`` on your manager, be sure to either
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call the method on the parent (using ``super()``) or do the appropriate
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handling of the ``_db`` attribute on the manager (a string containing the name
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of the database to use).
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For example, if you want to return a custom ``QuerySet`` class from the
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``get_query_set`` method, you could do this::
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class MyManager(models.Manager):
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def get_query_set(self):
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qs = CustomQuerySet(self.model)
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if self._db is not None:
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qs = qs.using(self._db)
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return qs
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Exposing multiple databases in Django's admin interface
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=======================================================
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Django's admin doesn't have any explicit support for multiple
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databases. If you want to provide an admin interface for a model on a
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database other than ``default``, you'll need to write custom
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:class:`~django.contrib.admin.ModelAdmin` classes that will direct the
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admin to use a specific database for content.
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``ModelAdmin`` objects have four methods that require customization for
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multiple-database support::
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class MultiDBModelAdmin(admin.ModelAdmin):
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# A handy constant for the name of the alternate database.
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using = 'other'
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def save_model(self, request, obj, form, change):
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# Tell Django to save objects to the 'other' database.
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obj.save(using=self.using)
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def queryset(self, request):
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# Tell Django to look for objects on the 'other' database.
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return super(MultiDBModelAdmin, self).queryset(request).using(self.using)
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def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
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# Tell Django to populate ForeignKey widgets using a query
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# on the 'other' database.
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return super(MultiDBModelAdmin, self).formfield_for_foreignkey(db_field, request=request, using=self.using, **kwargs)
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def formfield_for_manytomany(self, db_field, request=None, **kwargs):
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# Tell Django to populate ManyToMany widgets using a query
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# on the 'other' database.
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return super(MultiDBModelAdmin, self).formfield_for_manytomany(db_field, request=request, using=self.using, **kwargs)
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The implementation provided here implements a multi-database strategy where
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all objects of a given type are stored on a specific database (e.g.,
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all ``User`` objects are in the ``other`` database). If your usage of
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multiple databases is more complex, your ``ModelAdmin`` will need to reflect
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that strategy.
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Inlines can be handled in a similar fashion. They require three customized methods::
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class MultiDBTabularInline(admin.TabularInline):
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using = 'other'
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def queryset(self, request):
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# Tell Django to look for inline objects on the 'other' database.
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return super(MultiDBTabularInline, self).queryset(request).using(self.using)
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def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
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# Tell Django to populate ForeignKey widgets using a query
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# on the 'other' database.
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return super(MultiDBTabularInline, self).formfield_for_foreignkey(db_field, request=request, using=self.using, **kwargs)
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def formfield_for_manytomany(self, db_field, request=None, **kwargs):
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# Tell Django to populate ManyToMany widgets using a query
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# on the 'other' database.
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return super(MultiDBTabularInline, self).formfield_for_manytomany(db_field, request=request, using=self.using, **kwargs)
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Once you've written your model admin definitions, they can be registered with
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any ``Admin`` instance::
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from django.contrib import admin
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# Specialize the multi-db admin objects for use with specific models.
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class BookInline(MultiDBTabularInline):
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model = Book
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class PublisherAdmin(MultiDBModelAdmin):
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inlines = [BookInline]
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admin.site.register
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admin.site.register(Author, MultiDBModelAdmin)
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admin.site.register(Publisher, PublisherAdmin)
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othersite = admin.Site('othersite')
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othersite.register(Publisher, MultiDBModelAdmin)
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This example sets up two admin sites. On the first site, the
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``Author`` and ``Publisher`` objects are exposed; ``Publisher``
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objects have an tabular inline showing books published by that
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publisher. The second site exposes just publishers, without the
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inlines.
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