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

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@ -1,7 +1,7 @@
.. _topics-db-multi-db:
==================
Multiple Databases
Multiple databases
==================
.. versionadded:: 1.2
@ -9,7 +9,7 @@ Multiple Databases
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, some additional steps must be taken.
databases, you'll need to take some additional steps.
Defining your databases
=======================
@ -23,11 +23,11 @@ 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 you have can have
whatever alias you choose.
named ``'default'``. Any additional databases can have whatever alias
you choose.
The following is an example ``settings.py`` snippet defining two
databases - a default Postgres database, and a MySQL database called
databases -- a default PostgreSQL database and a MySQL database called
``users``:
.. code-block:: python
@ -48,7 +48,7 @@ databases - a default Postgres database, and a MySQL database called
}
If you attempt to access a database that you haven't defined in your
:setting:`DATABASES` setting then Django will raise a
:setting:`DATABASES` setting, Django will raise a
``django.db.utils.ConnectionDoesNotExist`` exception.
Synchronizing your databases
@ -57,7 +57,7 @@ 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
synchronize a different database. So, to synchronize all models onto
all databases in our example, you would need to call::
$ ./manage.py syncdb
@ -66,67 +66,74 @@ all databases in our example, you would need to call::
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
allows you to prevent a specific application or applications from
lets you prevent a specific application or applications from
being synchronized. For example, if you don't want the ``sales``
application to be on the ``users`` database, you could run::
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,
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.
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 the :djadminopt:`--database` to control
the database that is used.
on one database at a time, using :djadminopt:`--database` to control
the database used.
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``. ``using()`` takes a single argument: the alias of the
database on which you want to run the query. For example:
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...
# This will run on the 'default' database.
>>> Author.objects.all()
# So will this...
# So will this.
>>> Author.objects.using('default').all()
# This will run on the 'other' database
# This will run on the 'other' database.
>>> Author.objects.using('other').all()
Select a database to save to
============================
Selecting a database for ``save()``
===================================
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 save the user
by calling::
Use the ``using`` keyword to ``Model.save()`` to specify to which database the
data should be saved.
>>> user_obj.save(using='legacy_users')
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 have saved an instance to one database, it might be tempting to use
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') # (1)
# some other processing ...
>>> p.save(using='second') # (2)
>>> p.save(using='first') # (statement 1)
>>> p.save(using='second') # (statement 2)
In statement 1, a new Person object is saved to the ``first``
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``.
@ -135,29 +142,26 @@ 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.
copied to the new database.
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.
``second`` database, the existing object in the ``second`` database
will be overridden 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::
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')
# some other processing ...
>>> p.pk = None # Clear the PK
>>> p.save(using='second') # Write a completely new object
>>> p.pk = None # Clear the primary key.
>>> 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``::
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')
# some other processing ...
>>> p.save(using='second', force_insert=True)
This will ensure that the person named ``Fred`` will have the same
@ -165,50 +169,54 @@ 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
================================
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::
>>> user_obj = User.objects.using('legacy_users').get(username='fred')
>>> user_obj.delete() # will delete from the `legacy_users` database
>>> u = User.objects.using('legacy_users').get(username='fred')
>>> u.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::
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
==========================================
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 direct
a manager to use a specific database chain ``using()``. If you call
``using()``, you won't have access to any of the methods on the
manager.
Use the ``db_manager()`` method on managers to give managers access to a
non-default database.
To overcome this limitation, managers provide a ``db_manager()``
method. This method returns a copy of the *manager* bound to that
specific database. So, if you want to load an object using it's
natural key (using the ``get_by_natural_key()`` method on the manager,
you can call::
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::
>>> Book.objects.db_manager("other").get_by_natural_key(...)
User.objects.db_manager('new_users').create_user(...)
If you are overriding ``get_query_set()`` on your manager you must be sure to
either, call the method on the parent (using ``super()``), or 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::
``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:
@ -220,67 +228,66 @@ 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 need to write custom
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.
There are four methods that require customization on a ModelAdmin
object::
``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
# 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
# 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
# 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
# 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
# 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
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 on the ``other`` database). If your usage of
multi-db is more complex, your ModelAdmin will need to reflect that
strategy.
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 just three
customized methods::
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
# 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
# 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
# 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::
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
# Specialize the multi-db admin objects for use with specific models.
class BookInline(MultiDBTabularInline):
model = Book