---
title: "複数のデータベース"
version: 4.2
locale: ja
source: https://docs.djangoproject.com/ja/4.2/topics/db/multi-db/
canonical: https://djangodocs.dev/ja/4.2/topics/db/multi-db/
---
# 複数のデータベース

ここでは、Djangoがサポートする複数データベースの扱いについて説明します。残りのDjangoのドキュメントの大部分は、あなたが一つのデータベース扱うことを前提としています。複数のデータベースを使いたいなら、いくつかの追加ステップを踏む必要があります。

> **See also**
>
> 複数のデータベースを使用したテストに付いての情報は、[マルチデータベースのサポート](/ja/4.2/topics/testing/tools/#testing-multi-db) を参照してください。

## データベースを定義する

Djangoでデータベースを使うひとつめのステップは、利用するデータベースサーバーをDjangoに教えることです。これは、 [`DATABASES`](/ja/4.2/ref/settings/#std-setting-DATABASES) の設定で行います。この設定は、データベースへのエイリアスです。Djangoを通してデータベースを参照するためのデータベースエイリアスを、辞書にマッピングします。この辞書の設定の詳細は、 [`DATABASES`](/ja/4.2/ref/settings/#std-setting-DATABASES) ドキュメントで説明しています。

データベースには、選択した任意のエイリアスを付けることができます。しかし、エイリアス `default` は特別な意味があります。Django は、他のデータベースが選択されていない場合、`default` というエイリアスを持つデータベースを使用します。

以下に示すのは、2つのデータベース、すなわちデフォルトの PostgreSQL データベースと `users` という名前の MySQL データベースを定義した `settings.py` スニペットの例です。

```
DATABASES = {
    "default": {
        "NAME": "app_data",
        "ENGINE": "django.db.backends.postgresql",
        "USER": "postgres_user",
        "PASSWORD": "s3krit",
    },
    "users": {
        "NAME": "user_data",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_user",
        "PASSWORD": "priv4te",
    },
}
```

`default` データベースという概念がプロジェクトのコンテキストで意味をなさない場合、使用したいデータベースを注意深く常に指定する必要があります。Django は `default` データベースエントリーが定義されていることを要求しますが、使用しない場合はパラメータのディクショナリを空白のままにできます。これを行うには、使用している contrib とサードパーティを含むすべてのアプリのモデルで [`DATABASE_ROUTERS`](/ja/4.2/ref/settings/#std-setting-DATABASE_ROUTERS) をセットアップしなければなりません。以下は、`default` エントリーを意図的に空のままにして、2つの default ではないデータベースを定義している `settings.py` スニペットの例です。

```
DATABASES = {
    "default": {},
    "users": {
        "NAME": "user_data",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_user",
        "PASSWORD": "superS3cret",
    },
    "customers": {
        "NAME": "customer_data",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_cust",
        "PASSWORD": "veryPriv@ate",
    },
}
```

[`DATABASES`](/ja/4.2/ref/settings/#std-setting-DATABASES) 設定で定義されていないデータベースへのアクセスを試みた場合、Django は `django.utils.connection.ConnectionDoesNotExist` 例外を発生されます。

## データベースを同期する

The [`migrate`](/ja/4.2/ref/django-admin/#django-admin-migrate) management command operates on one database at a
time. By default, it operates on the `default` database, but by
providing the [`--database`](/ja/4.2/ref/django-admin/#cmdoption-migrate-database) option, you can tell it
to synchronize a different database. So, to synchronize all models onto
all databases in the first example above, you would need to call:

```shell
$ ./manage.py migrate
$ ./manage.py migrate --database=users
```

If you don't want every application to be synchronized onto a
particular database, you can define a [database
router](#topics-db-multi-db-routing) that implements a policy
constraining the availability of particular models.

If, as in the second example above, you've left the `default` database empty,
you must provide a database name each time you run [`migrate`](/ja/4.2/ref/django-admin/#django-admin-migrate). Omitting
the database name would raise an error. For the second example:

```shell
$ ./manage.py migrate --database=users
$ ./manage.py migrate --database=customers
```

### 他の管理コマンドを使用する

Most other `django-admin` commands that interact with the database operate in
the same way as [`migrate`](/ja/4.2/ref/django-admin/#django-admin-migrate) -- they only ever operate on one database at
a time, using `--database` to control the database used.

An exception to this rule is the [`makemigrations`](/ja/4.2/ref/django-admin/#django-admin-makemigrations) command. It
validates the migration history in the databases to catch problems with the
existing migration files (which could be caused by editing them) before
creating new migrations. By default, it checks only the `default` database,
but it consults the [`allow_migrate()`](#allow_migrate) method of [routers](#topics-db-multi-db-routing) if any are installed.

## 自動的なデータベースのルーティング

The easiest way to use multiple databases is to set up a database
routing scheme. The default routing scheme ensures that objects remain
'sticky' to their original database (i.e., an object retrieved from
the `foo` database will be saved on the same database). The default
routing scheme ensures that if a database isn't specified, all queries
fall back to the `default` database.

You don't have to do anything to activate the default routing scheme
-- it is provided 'out of the box' on every Django project. However,
if you want to implement more interesting database allocation
behaviors, you can define and install your own database routers.

### データベース ルーター

A database Router is a class that provides up to four methods:

#### `db_for_read(model, **hints)`

Suggest the database that should be used for read operations for
objects of type `model`.

If a database operation is able to provide any additional
information that might assist in selecting a database, it will be
provided in the `hints` dictionary. Details on valid hints are
provided [below](#topics-db-multi-db-hints).

Returns `None` if there is no suggestion.

#### `db_for_write(model, **hints)`

Suggest the database that should be used for writes of objects of
type Model.

If a database operation is able to provide any additional
information that might assist in selecting a database, it will be
provided in the `hints` dictionary. Details on valid hints are
provided [below](#topics-db-multi-db-hints).

Returns `None` if there is no suggestion.

#### `allow_relation(obj1, obj2, **hints)`

Return `True` if a relation between `obj1` and `obj2` should be
allowed, `False` if the relation should be prevented, or `None` if
the router has no opinion. This is purely a validation operation,
used by foreign key and many to many operations to determine if a
relation should be allowed between two objects.

If no router has an opinion (i.e. all routers return `None`), only
relations within the same database are allowed.

#### `allow_migrate(db, app_label, model_name=None, **hints)`

Determine if the migration operation is allowed to run on the database with
alias `db`. Return `True` if the operation should run, `False` if it
shouldn't run, or `None` if the router has no opinion.

The `app_label` positional argument is the label of the application
being migrated.

`model_name` is set by most migration operations to the value of
`model._meta.model_name` (the lowercased version of the model
`__name__`) of the model being migrated. Its value is `None` for the
[`RunPython`](/ja/4.2/ref/migration-operations/#django.db.migrations.operations.RunPython) and
[`RunSQL`](/ja/4.2/ref/migration-operations/#django.db.migrations.operations.RunSQL) operations unless they
provide it using hints.

`hints` are used by certain operations to communicate additional
information to the router.

When `model_name` is set, `hints` normally contains the model class
under the key `'model'`. Note that it may be a [historical model](/ja/4.2/topics/migrations/#historical-models), and thus not have any custom attributes, methods, or
managers. You should only rely on `_meta`.

This method can also be used to determine the availability of a model on a
given database.

[`makemigrations`](/ja/4.2/ref/django-admin/#django-admin-makemigrations) always creates migrations for model changes, but
if `allow_migrate()` returns `False`, any migration operations for the
`model_name` will be silently skipped when running [`migrate`](/ja/4.2/ref/django-admin/#django-admin-migrate) on
the `db`. Changing the behavior of `allow_migrate()` for models that
already have migrations may result in broken foreign keys, extra tables,
or missing tables. When [`makemigrations`](/ja/4.2/ref/django-admin/#django-admin-makemigrations) verifies the migration
history, it skips databases where no app is allowed to migrate.

A router doesn't have to provide *all* these methods -- it may omit one
or more of them. If one of the methods is omitted, Django will skip
that router when performing the relevant check.

#### ヒント

The hints received by the database router can be used to decide which
database should receive a given request.

At present, the only hint that will be provided is `instance`, an
object instance that is related to the read or write operation that is
underway. This might be the instance that is being saved, or it might
be an instance that is being added in a many-to-many relation. In some
cases, no instance hint will be provided at all. The router checks for
the existence of an instance hint, and determine if that hint should be
used to alter routing behavior.

### ルーターを使用する

Database routers are installed using the [`DATABASE_ROUTERS`](/ja/4.2/ref/settings/#std-setting-DATABASE_ROUTERS)
setting. This setting defines a list of class names, each specifying a
router that should be used by the base router
(`django.db.router`).

The base router is used by Django's database operations to allocate
database usage. Whenever a query needs to know which database to use,
it calls the base router, providing a model and a hint (if
available). The base router tries each router class in turn until one returns
a database suggestion. If no routers return a suggestion, the base router tries
the current [`instance._state.db`](/ja/4.2/ref/models/instances/#django.db.models.Model._state) of the hint instance. If no hint instance
was provided, or [`instance._state.db`](/ja/4.2/ref/models/instances/#django.db.models.Model._state) is
`None`, the base router will allocate the `default` database.

### An example

> **例としてのみ使用してください！**
>
> この例は、ルーターのインフラストラクチャを使用してデータベースの使用状況を変更する方法のデモを示すことだけを目的としています。ルーターの使用方法を紹介するために、いくつかの複雑な問題を意図的に無視しています。
>
> この例は、`myapp` 内のモデルのいずれかに `other` データベースの外部にあるモデルとのリレーションが含まれている場合には機能しません。[クロスデータベース リレーション](#no-cross-database-relations) を使うと、現時点では Django が処理できない参照整合性の問題が発生します。
>
> The primary/replica (referred to as master/slave by some databases)
> configuration described is also flawed -- it
> doesn't provide any solution for handling replication lag (i.e.,
> query inconsistencies introduced because of the time taken for a
> write to propagate to the replicas). It also doesn't consider the
> interaction of transactions with the database utilization strategy.

So - what does this mean in practice? Let's consider another sample
configuration. This one will have several databases: one for the
`auth` application, and all other apps using a primary/replica setup
with two read replicas. Here are the settings specifying these
databases:

```
DATABASES = {
    "default": {},
    "auth_db": {
        "NAME": "auth_db_name",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_user",
        "PASSWORD": "swordfish",
    },
    "primary": {
        "NAME": "primary_name",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_user",
        "PASSWORD": "spam",
    },
    "replica1": {
        "NAME": "replica1_name",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_user",
        "PASSWORD": "eggs",
    },
    "replica2": {
        "NAME": "replica2_name",
        "ENGINE": "django.db.backends.mysql",
        "USER": "mysql_user",
        "PASSWORD": "bacon",
    },
}
```

Now we'll need to handle routing. First we want a router that knows to
send queries for the `auth` and `contenttypes` apps to `auth_db`
(`auth` models are linked to `ContentType`, so they must be stored in the
same database):

```
class AuthRouter:
    """
    A router to control all database operations on models in the
    auth and contenttypes applications.
    """

    route_app_labels = {"auth", "contenttypes"}

    def db_for_read(self, model, **hints):
        """
        Attempts to read auth and contenttypes models go to auth_db.
        """
        if model._meta.app_label in self.route_app_labels:
            return "auth_db"
        return None

    def db_for_write(self, model, **hints):
        """
        Attempts to write auth and contenttypes models go to auth_db.
        """
        if model._meta.app_label in self.route_app_labels:
            return "auth_db"
        return None

    def allow_relation(self, obj1, obj2, **hints):
        """
        Allow relations if a model in the auth or contenttypes apps is
        involved.
        """
        if (
            obj1._meta.app_label in self.route_app_labels
            or obj2._meta.app_label in self.route_app_labels
        ):
            return True
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        Make sure the auth and contenttypes apps only appear in the
        'auth_db' database.
        """
        if app_label in self.route_app_labels:
            return db == "auth_db"
        return None
```

And we also want a router that sends all other apps to the
primary/replica configuration, and randomly chooses a replica to read
from:

```
import random

class PrimaryReplicaRouter:
    def db_for_read(self, model, **hints):
        """
        Reads go to a randomly-chosen replica.
        """
        return random.choice(["replica1", "replica2"])

    def db_for_write(self, model, **hints):
        """
        Writes always go to primary.
        """
        return "primary"

    def allow_relation(self, obj1, obj2, **hints):
        """
        Relations between objects are allowed if both objects are
        in the primary/replica pool.
        """
        db_set = {"primary", "replica1", "replica2"}
        if obj1._state.db in db_set and obj2._state.db in db_set:
            return True
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        All non-auth models end up in this pool.
        """
        return True
```

Finally, in the settings file, we add the following (substituting
`path.to.` with the actual Python path to the module(s) where the
routers are defined):

```
DATABASE_ROUTERS = ["path.to.AuthRouter", "path.to.PrimaryReplicaRouter"]
```

The order in which routers are processed is significant. Routers will
be queried in the order they are listed in the
[`DATABASE_ROUTERS`](/ja/4.2/ref/settings/#std-setting-DATABASE_ROUTERS) setting. In this example, the
`AuthRouter` is processed before the `PrimaryReplicaRouter`, and as a
result, decisions concerning the models in `auth` are processed
before any other decision is made. If the [`DATABASE_ROUTERS`](/ja/4.2/ref/settings/#std-setting-DATABASE_ROUTERS)
setting listed the two routers in the other order,
`PrimaryReplicaRouter.allow_migrate()` would be processed first. The
catch-all nature of the PrimaryReplicaRouter implementation would mean
that all models would be available on all databases.

With this setup installed, and all databases migrated as per
[データベースを同期する](#synchronizing-multiple-databases), lets run some Django code:

```pycon
>>> # This retrieval will be performed on the 'auth_db' database
>>> fred = User.objects.get(username="fred")
>>> fred.first_name = "Frederick"

>>> # This save will also be directed to 'auth_db'
>>> fred.save()

>>> # These retrieval will be randomly allocated to a replica database
>>> dna = Person.objects.get(name="Douglas Adams")

>>> # A new object has no database allocation when created
>>> mh = Book(title="Mostly Harmless")

>>> # This assignment will consult the router, and set mh onto
>>> # the same database as the author object
>>> mh.author = dna

>>> # This save will force the 'mh' instance onto the primary database...
>>> mh.save()

>>> # ... but if we re-retrieve the object, it will come back on a replica
>>> mh = Book.objects.get(title="Mostly Harmless")
```

This example defined a router to handle interaction with models from the
`auth` app, and other routers to handle interaction with all other apps. If
you left your `default` database empty and don't want to define a catch-all
database router to handle all apps not otherwise specified, your routers must
handle the names of all apps in [`INSTALLED_APPS`](/ja/4.2/ref/settings/#std-setting-INSTALLED_APPS) before you migrate.
See [Behavior of contrib apps](#contrib-app-multiple-databases) for information about contrib apps
that must be together in one database.

## データベースを手動で選択する

Django also provides an API that allows you to maintain complete control
over database usage in your code. A manually specified database allocation
will take priority over a database allocated by a router.

### `QuerySet` のためのデータベースを手動で選択する

You can select the database for a `QuerySet` at any point in the
`QuerySet` "chain." 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:

```pycon
>>> # This will run on the 'default' database.
>>> Author.objects.all()

>>> # So will this.
>>> Author.objects.using("default")

>>> # This will run on the 'other' database.
>>> Author.objects.using("other")
```

### `save()` のためのデータベースを選択する

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:

```pycon
>>> my_object.save(using="legacy_users")
```

If you don't specify `using`, the `save()` method will save into
the default database allocated by the routers.

#### オブジェクトを1つのデータベースからもう1つのデータベースに移動する

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.

以下の例を考えてみてください。

```pycon
>>> 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 an 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:

```pycon
>>> 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 an SQL `INSERT`:

```pycon
>>> 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.

### 削除が行われるデータベースを選択する

デフォルトでは、既存のオブジェクトを削除する呼び出しは、最初にオブジェクトを取得するために使用したのと同じデータベース上で実行されます。

```pycon
>>> u = User.objects.using("legacy_users").get(username="fred")
>>> u.delete()  # will delete from the `legacy_users` database
```

モデルを削除するデータベースを指定するには、`using` キーワード引数を `Model.delete()` メソッドに渡します。この引数は、`save()` の `using` キーワード引数と同じように機能します。

For example, if you're migrating a user from the `legacy_users`
database to the `new_users` database, you might use these commands:

```pycon
>>> 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_queryset()` with multiple databases

If you're overriding `get_queryset()` 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_queryset` method, you could do this:

```
class MyManager(models.Manager):
    def get_queryset(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 that specified by your router chain, you'll
need to write custom [`ModelAdmin`](/ja/4.2/ref/contrib/admin/#django.contrib.admin.ModelAdmin) classes
that will direct the admin to use a specific database for content.

`ModelAdmin` objects have the following 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 delete_model(self, request, obj):
        # Tell Django to delete objects from the 'other' database
        obj.delete(using=self.using)

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

    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        # Tell Django to populate ForeignKey widgets using a query
        # on the 'other' database.
        return super().formfield_for_foreignkey(
            db_field, request, using=self.using, **kwargs
        )

    def formfield_for_manytomany(self, db_field, request, **kwargs):
        # Tell Django to populate ManyToMany widgets using a query
        # on the 'other' database.
        return super().formfield_for_manytomany(
            db_field, 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.

[`InlineModelAdmin`](/ja/4.2/ref/contrib/admin/#django.contrib.admin.InlineModelAdmin) objects can be handled in a
similar fashion. They require three customized methods:

```
class MultiDBTabularInline(admin.TabularInline):
    using = "other"

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

    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        # Tell Django to populate ForeignKey widgets using a query
        # on the 'other' database.
        return super().formfield_for_foreignkey(
            db_field, request, using=self.using, **kwargs
        )

    def formfield_for_manytomany(self, db_field, request, **kwargs):
        # Tell Django to populate ManyToMany widgets using a query
        # on the 'other' database.
        return super().formfield_for_manytomany(
            db_field, 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(Author, MultiDBModelAdmin)
admin.site.register(Publisher, PublisherAdmin)

othersite = admin.AdminSite("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 a tabular inline showing books published by that
publisher. The second site exposes just publishers, without the
inlines.

## Using raw cursors with multiple databases

If you are using more than one database you can use
`django.db.connections` to obtain the connection (and cursor) for a
specific database. `django.db.connections` is a dictionary-like
object that allows you to retrieve a specific connection using its
alias:

```
from django.db import connections

with connections["my_db_alias"].cursor() as cursor:
    ...
```

## Limitations of multiple databases

### Cross-database relations

Django doesn't currently provide any support for foreign key or
many-to-many relationships spanning multiple databases. If you
have used a router to partition models to different databases,
any foreign key and many-to-many relationships defined by those
models must be internal to a single database.

This is because of referential integrity. In order to maintain a
relationship between two objects, Django needs to know that the
primary key of the related object is valid. If the primary key is
stored on a separate database, it's not possible to easily evaluate
the validity of a primary key.

If you're using Postgres, Oracle, or MySQL with InnoDB, this is
enforced at the database integrity level -- database level key
constraints prevent the creation of relations that can't be validated.

However, if you're using SQLite or MySQL with MyISAM tables, there is
no enforced referential integrity; as a result, you may be able to
'fake' cross database foreign keys. However, this configuration is not
officially supported by Django.

### Behavior of contrib apps

Several contrib apps include models, and some apps depend on others. Since
cross-database relationships are impossible, this creates some restrictions on
how you can split these models across databases:

- each one of `contenttypes.ContentType`, `sessions.Session` and
  `sites.Site` can be stored in any database, given a suitable router.
- `auth` models — `User`, `Group` and `Permission` — are linked
  together and linked to `ContentType`, so they must be stored in the same
  database as `ContentType`.
- `admin` depends on `auth`, so its models must be in the same database
  as `auth`.
- `flatpages` and `redirects` depend on `sites`, so their models must be
  in the same database as `sites`.

In addition, some objects are automatically created just after
[`migrate`](/ja/4.2/ref/django-admin/#django-admin-migrate) creates a table to hold them in a database:

- a default `Site`,
- a `ContentType` for each model (including those not stored in that
  database),
- the `Permission`s for each model (including those not stored in that
  database).

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 to write a
[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**
>
> If you're synchronizing content types to more than one database, be aware
> that their primary keys may not match across databases. This may result in
> data corruption or data loss.