---
title: "多対多 (many-to-many) 関係"
version: 6.1
locale: ja
source: https://docs.djangoproject.com/ja/6.1/topics/db/examples/many_to_many/
canonical: https://djangodocs.dev/ja/6.1/topics/db/examples/many_to_many/
---
# 多対多 (many-to-many) 関係

多対多の関係を定義するには、[`ManyToManyField`](/ja/6.1/ref/models/fields/#django.db.models.ManyToManyField) を使用します。

この例では、一つの `Article` は複数の `Publication` オブジェクトに掲載することができ、一つの `Publication` は複数の `Article` オブジェクトを持ちます:

```python
from django.db import models

class Publication(models.Model):
    title = models.CharField(max_length=30)

    class Meta:
        ordering = ["title"]

    def __str__(self):
        return self.title

class Article(models.Model):
    headline = models.CharField(max_length=100)
    publications = models.ManyToManyField(Publication)

    class Meta:
        ordering = ["headline"]

    def __str__(self):
        return self.headline
```

以下は、Python API の機能を使って実行できる操作の例です。

`Publication` インスタンスをいくつか作ります：

```pycon
>>> p1 = Publication(title="The Python Journal")
>>> p1.save()
>>> p2 = Publication(title="Science News")
>>> p2.save()
>>> p3 = Publication(title="Science Weekly")
>>> p3.save()
```

`Article` を一つ作ります：

```pycon
>>> a1 = Article(headline="Django lets you build web apps easily")
```

保存するまでは `Publication` と関連付けることはできません：

```pycon
>>> a1.publications.add(p1)
Traceback (most recent call last):
...
ValueError: "<Article: Django lets you build web apps easily>" needs to have a value for field "id" before this many-to-many relationship can be used.
```

保存しましょう！

```pycon
>>> a1.save()
```

`Article` と `Publication` を関連付けます：

```pycon
>>> a1.publications.add(p1)
```

`Article` をもう一つ作り、それが publications に表示されるようセットします：

```pycon
>>> a2 = Article(headline="NASA uses Python")
>>> a2.save()
>>> a2.publications.add(p1, p2)
>>> a2.publications.add(p3)
```

2回めの追加をしてしまっても大丈夫です、関係が重複することはありません：

```pycon
>>> a2.publications.add(p3)
```

間違った型のオブジェクトを追加すると [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError) が発生します:

```pycon
>>> a2.publications.add(a1)
Traceback (most recent call last):
...
TypeError: 'Publication' instance expected
```

`Article` に `Publication` をワンステップで作成・追加するには [`create()`](/ja/6.1/ref/models/relations/#django.db.models.fields.related.RelatedManager.create) を使います:

```pycon
>>> new_publication = a2.publications.create(title="Highlights for Children")
```

`Article` オブジェクトはリレーション先の `Publication` オブジェクトにアクセスできます:

```pycon
>>> a1.publications.all()
<QuerySet [<Publication: The Python Journal>]>
>>> a2.publications.all()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
```

`Publication` オブジェクトはリレーション先の `Article` オブジェクトにアクセスできます:

```pycon
>>> p2.article_set.all()
<QuerySet [<Article: NASA uses Python>]>
>>> p1.article_set.all()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Publication.objects.get(id=4).article_set.all()
<QuerySet [<Article: NASA uses Python>]>
```

多対多の関係は [リレーションシップを横断するルックアップ](/ja/6.1/topics/db/queries/#lookups-that-span-relationships) を使ってクエリできます:

```pycon
>>> Article.objects.filter(publications__id=1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications__pk=1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications=1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications=p1)
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>

>>> Article.objects.filter(publications__title__startswith="Science")
<QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>

>>> Article.objects.filter(publications__title__startswith="Science").distinct()
<QuerySet [<Article: NASA uses Python>]>
```

[`count()`](/ja/6.1/ref/models/querysets/#django.db.models.query.QuerySet.count) 関数は [`distinct()`](/ja/6.1/ref/models/querysets/#django.db.models.query.QuerySet.distinct) も尊重します:

```pycon
>>> Article.objects.filter(publications__title__startswith="Science").count()
2

>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()
1

>>> Article.objects.filter(publications__in=[1, 2]).distinct()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
>>> Article.objects.filter(publications__in=[p1, p2]).distinct()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>
```

逆の多対多クエリもサポートされています (つまり、 [`ManyToManyField`](/ja/6.1/ref/models/fields/#django.db.models.ManyToManyField) を持たないテーブルから開始できます):

```pycon
>>> Publication.objects.filter(id=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(pk=1)
<QuerySet [<Publication: The Python Journal>]>

>>> Publication.objects.filter(article__headline__startswith="NASA")
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>

>>> Publication.objects.filter(article__id=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(article__pk=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(article=1)
<QuerySet [<Publication: The Python Journal>]>
>>> Publication.objects.filter(article=a1)
<QuerySet [<Publication: The Python Journal>]>

>>> Publication.objects.filter(article__in=[1, 2]).distinct()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
>>> Publication.objects.filter(article__in=[a1, a2]).distinct()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>
```

リレーション先のアイテムの除外も期待通りに動作します（SQLは少し複雑ですが）:

```pycon
>>> Article.objects.exclude(publications=p2)
<QuerySet [<Article: Django lets you build web apps easily>]>
```

`Publication` を削除すると、関連する `Article` インスタンスにはアクセスできなくなります：

```pycon
>>> p1.delete()
>>> Publication.objects.all()
<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>
>>> a1 = Article.objects.get(pk=1)
>>> a1.publications.all()
<QuerySet []>
```

`Article` を削除すると、関連する `Publication` インスタンスにはアクセスできなくなります：

```pycon
>>> a2.delete()
>>> Article.objects.all()
<QuerySet [<Article: Django lets you build web apps easily>]>
>>> p2.article_set.all()
<QuerySet []>
```

多対多のリレーションシップの「反対側の」端を経由した追加:

```pycon
>>> a4 = Article(headline="NASA finds intelligent life on Earth")
>>> a4.save()
>>> p2.article_set.add(a4)
>>> p2.article_set.all()
<QuerySet [<Article: NASA finds intelligent life on Earth>]>
>>> a4.publications.all()
<QuerySet [<Publication: Science News>]>
```

キーワードを使った反対側の端からの追加:

```pycon
>>> new_article = p2.article_set.create(headline="Oxygen-free diet works wonders")
>>> p2.article_set.all()
<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
>>> a5 = p2.article_set.all()[1]
>>> a5.publications.all()
<QuerySet [<Publication: Science News>]>
```

`Article` から `Publication` を削除する:

```pycon
>>> a4.publications.remove(p2)
>>> p2.article_set.all()
<QuerySet [<Article: Oxygen-free diet works wonders>]>
>>> a4.publications.all()
<QuerySet []>
```

反対側からも:

```pycon
>>> p2.article_set.remove(a5)
>>> p2.article_set.all()
<QuerySet []>
>>> a5.publications.all()
<QuerySet []>
```

リレーションのセットも指定できます:

```pycon
>>> a4.publications.all()
<QuerySet [<Publication: Science News>]>
>>> a4.publications.set([p3])
>>> a4.publications.all()
<QuerySet [<Publication: Science Weekly>]>
```

リレーションのセットを削除することもできます:

```pycon
>>> p2.article_set.clear()
>>> p2.article_set.all()
<QuerySet []>
```

反対側からも削除できます:

```pycon
>>> p2.article_set.add(a4, a5)
>>> p2.article_set.all()
<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>
>>> a4.publications.all()
<QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>
>>> a4.publications.clear()
>>> a4.publications.all()
<QuerySet []>
>>> p2.article_set.all()
<QuerySet [<Article: Oxygen-free diet works wonders>]>
```

削除した `Article` と `Publication` を再作成する:

```pycon
>>> p1 = Publication(title="The Python Journal")
>>> p1.save()
>>> a2 = Article(headline="NASA uses Python")
>>> a2.save()
>>> a2.publications.add(p1, p2, p3)
```

いくつかの `Publication` インスタンスを一括削除すると、削除された publications への参照は関連するエントリーから除外されるようになります：

```pycon
>>> Publication.objects.filter(title__startswith="Science").delete()
>>> Publication.objects.all()
<QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>
>>> Article.objects.all()
<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA finds intelligent life on Earth>, <Article: NASA uses Python>, <Article: Oxygen-free diet works wonders>]>
>>> a2.publications.all()
<QuerySet [<Publication: The Python Journal>]>
```

複数の記事を一括削除 - 削除したオブジェクトへの参照は削除されます:

```pycon
>>> q = Article.objects.filter(headline__startswith="Django")
>>> print(q)
<QuerySet [<Article: Django lets you build web apps easily>]>
>>> q.delete()
```

[`delete()`](/ja/6.1/ref/models/querysets/#django.db.models.query.QuerySet.delete) の後は、 [`QuerySet`](/ja/6.1/ref/models/querysets/#django.db.models.query.QuerySet) のキャッシュをクリアする必要があります。参照されたオブジェクトは削除されます:

```pycon
>>> print(q)
<QuerySet []>
>>> p1.article_set.all()
<QuerySet [<Article: NASA uses Python>]>
```