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ドキュメントのバージョン: 5.1

多対多 (many-to-many) 関係¶

多対多の関係を定義するには、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:

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。

いくつかの 出版物 を作成する:

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

一つの Article を作成する:

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

保存されるまでは Publication と関連付けることはできません:

>>> 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！

>>> a1.save()

Article と Publication を関連付ける:

>>> a1.publications.add(p1)

別の Article を作成し、それを Publications に表示するようにセットする:

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

2回の追加はOKで、関係が重複することはありません:

>>> a2.publications.add(p3)

間違った型のオブジェクトを追加すると TypeError が発生します:

>>> 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() を使います:

>>> 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:

>>> 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:

>>> 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を横断するルックアップを使ってクエリできます:

>>> 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() 関数は distinct() も尊重します:

>>> 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 を持たないテーブルから開始できます):

>>> 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>]>

リレーション先項目の除外も期待通りに動作します（SQLは少し複雑ですが）:

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

もし Publication を削除すると、その Articles にはアクセスできなくなります:

>>> 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 []>

もし Article を削除すると、その Publications にはアクセスできなくなります:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

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

反対側からも削除できます:

>>> 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:

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

複数の Publications を一括削除 - 削除された出版物への参照は削除されます:

>>> 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:

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

delete() の後は、 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:

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

モデルリレーション API の使用例

多対一 (many-to-one) 関係

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多対多 (many-to-many) 関係¶

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