.. _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. If you want to interact with multiple databases, you'll need to take some additional steps. 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. Regardless of how many databases you have, you *must* have a database named ``'default'``. Any additional databases can have whatever alias you choose. The following is an example ``settings.py`` snippet defining two databases -- a default PostgreSQL database and a MySQL database called ``users``: .. code-block:: python DATABASES = { 'default': { 'NAME': 'app_data', 'ENGINE': 'django.db.backends.postgresql_psycopg2', 'USER': 'postgres_user', 'PASSWORD': 's3krit' }, 'users': { 'NAME': 'user_data' 'ENGINE': 'django.db.backends.mysql', 'USER': 'mysql_user', 'PASSWORD': 'priv4te' } } If you attempt to access a database that you haven't defined in your :setting:`DATABASES` setting, Django will raise a ``django.db.utils.ConnectionDoesNotExist`` exception. Synchronizing your databases ============================ The :djadmin:`syncdb` management command operates on one database at a time. By default, it operates on the ``default`` database, but by providing a :djadminopt:`--database` argument, you can tell syncdb to synchronize a different database. So, to synchronize all models onto all databases in our example, you would need to call:: $ ./manage.py syncdb $ ./manage.py syncdb --database=users If you don't want every application to be synchronized onto a particular database. you can specify the :djadminopt:`--exclude` argument to :djadmin:`syncdb`. The :djadminopt:`--exclude` option lets you prevent a specific application or applications from being synchronized. For example, if you don't want the ``sales`` application to be in the ``users`` database, you could run:: $ ./manage.py syncdb --database=users --exclude=sales 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 ------------------------------- The other ``django-admin.py`` commands that interact with the database operate in the same way as :djadmin:`syncdb` -- they only ever operate on one database at a time, using :djadminopt:`--database` to control the database used. Selecting a database for a ``QuerySet`` ======================================= You can select the database for a ``QuerySet`` at any point in the ``QuerySet`` "chain." Just call ``using()`` on the ``QuerySet`` to get another ``QuerySet`` that uses the specified database. ``using()`` takes a single argument: the alias of the database on which you want to run the query. For example: .. code-block:: python # This will run on the 'default' database. >>> Author.objects.all() # So will this. >>> Author.objects.using('default').all() # This will run on the 'other' database. >>> Author.objects.using('other').all() Selecting a database for ``save()`` =================================== Use the ``using`` keyword to ``Model.save()`` to specify to which database the data should be saved. For example, to save an object to the ``legacy_users`` database, you'd use this:: >>> my_object.save(using='legacy_users') If you don't specify ``using``, the ``save()`` method will always save into the default database. Moving an object from one database to another --------------------------------------------- If you've 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') # (statement 1) >>> p.save(using='second') # (statement 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 database. However, if the primary key of ``p`` is already in use on the ``second`` database, the existing object in the ``second`` database will be overridden when ``p`` is saved. You can avoid this in two ways. First, 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') >>> p.pk = None # Clear the primary key. >>> p.save(using='second') # Write a completely new object. The second option is to use the ``force_insert`` option to ``save()`` to ensure that Django does a SQL ``INSERT``:: >>> p = Person(name='Fred') >>> p.save(using='first') >>> 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. Selecting 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:: >>> u = User.objects.using('legacy_users').get(username='fred') >>> u.delete() # will delete from the `legacy_users` database To specify the database from which a model will be deleted, pass a ``using`` keyword argument to the ``Model.delete()`` method. This argument works just like the ``using`` keyword argument to ``save()``. For example, if you're migrating a user from the ``legacy_users`` database to the ``new_users`` database, you might use these commands:: >>> user_obj.save(using='new_users') >>> user_obj.delete(using='legacy_users') Using managers with multiple databases ====================================== Use the ``db_manager()`` method on managers to give managers access to a non-default database. For example, say you have a custom manager method that touches the database -- ``User.objects.create_user()``. Because ``create_user()`` is a manager method, not a ``QuerySet`` method, you can't do ``User.objects.using('new_users').create_user()``. (The ``create_user()`` method is only available on ``User.objects``, the manager, not on ``QuerySet`` objects derived from the manager.) The solution is to use ``db_manager()``, like this:: User.objects.db_manager('new_users').create_user(...) ``db_manager()`` returns a copy of the manager bound to the database you specify. Using ``get_query_set()`` with multiple databases ------------------------------------------------- If you're overriding ``get_query_set()`` on your manager, be sure to either call the method on the parent (using ``super()``) or do the appropriate handling of the ``_db`` attribute on the manager (a string containing the name of the database to use). For example, if you want 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 multiple databases. If you want to provide an admin interface for a model on a database other than ``default``, you'll need to write custom :class:`~django.contrib.admin.ModelAdmin` classes that will direct the admin to use a specific database for content. ``ModelAdmin`` objects have four methods that require customization for multiple-database support:: 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-database strategy where all objects of a given type are stored on a specific database (e.g., all ``User`` objects are in the ``other`` database). If your usage of multiple databases is more complex, your ``ModelAdmin`` will need to reflect that strategy. Inlines can be handled in a similar fashion. They require 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've 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.