{"title":"多対一 (many-to-one) 関係","version":"6.0","locale":"ja","docname":"topics/db/examples/many_to_one","url":"/ja/6.0/topics/db/examples/many_to_one/","canonical":"https://djangodocs.dev/ja/6.0/topics/db/examples/many_to_one/","summary":"多対一のリレーションシップを定義するには ForeignKey を使います。","html":"

多対一 (many-to-one) 関係Link to this heading#

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多対一のリレーションシップを定義するには ForeignKey を使います。

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この例では、 Reporter は複数の Article オブジェクトと関連付けることができますが、 Article は一つの Reporter オブジェクトしか持つことができません:

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Code
from django.db import models\n\n\nclass Reporter(models.Model):\n    first_name = models.CharField(max_length=30)\n    last_name = models.CharField(max_length=30)\n    email = models.EmailField()\n\n    def __str__(self):\n        return f"{self.first_name} {self.last_name}"\n\n\nclass Article(models.Model):\n    headline = models.CharField(max_length=100)\n    pub_date = models.DateField()\n    reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)\n\n    def __str__(self):\n        return self.headline\n\n    class Meta:\n        ordering = ["headline"]\n
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以下は、Python API の機能を使って実行できる操作の例です。

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複数の Reporter を作成する:

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Python console
>>> r = Reporter(first_name="John", last_name="Smith", email="john@example.com")\n>>> r.save()\n\n>>> r2 = Reporter(first_name="Paul", last_name="Jones", email="paul@example.com")\n>>> r2.save()\n
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Article を作成する:

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Python console
>>> from datetime import date\n>>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), reporter=r)\n>>> a.save()\n\n>>> a.reporter.id\n1\n\n>>> a.reporter\n<Reporter: John Smith>\n
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オブジェクトを外部キーリレーションシップに代入する前に保存する必要があることに注意してください。たとえば、保存されていない Reporter で Article を作成すると、次のように ValueError が発生します。

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Python console
>>> r3 = Reporter(first_name="John", last_name="Smith", email="john@example.com")\n>>> Article.objects.create(\n...     headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3\n... )\nTraceback (most recent call last):\n...\nValueError: save() prohibited to prevent data loss due to unsaved related object 'reporter'.\n
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Article オブジェクトはリレーション先オブジェクトにアクセスできます:

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Python console
>>> r = a.reporter\n
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Reporter オブジェクトを使って Article を作成する:

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Python console
>>> new_article = r.article_set.create(\n...     headline="John's second story", pub_date=date(2005, 7, 29)\n... )\n>>> new_article\n<Article: John's second story>\n>>> new_article.reporter\n<Reporter: John Smith>\n>>> new_article.reporter.id\n1\n
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新しい記事を作成する:

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Python console
>>> new_article2 = Article.objects.create(\n...     headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r\n... )\n>>> new_article2.reporter\n<Reporter: John Smith>\n>>> new_article2.reporter.id\n1\n>>> r.article_set.all()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n
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同じ記事を別の記事セットに追加する - 移動することを確認します:

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Python console
>>> r2.article_set.add(new_article2)\n>>> new_article2.reporter.id\n2\n>>> new_article2.reporter\n<Reporter: Paul Jones>\n
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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:

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Python console
>>> r.article_set.add(r2)\nTraceback (most recent call last):\n...\nTypeError: 'Article' instance expected, got <Reporter: Paul Jones>\n\n>>> r.article_set.all()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> r2.article_set.all()\n<QuerySet [<Article: Paul's story>]>\n\n>>> r.article_set.count()\n2\n\n>>> r2.article_set.count()\n1\n
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最後の例では、記事が John から Paul に移動していることに注意してください。

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関係マネージャはフィールドルックアップもサポートします。APIは必要な範囲まで自動的にリレーションシップをたどります。リレーションシップを区切るには2重アンダースコアを使用してください。これは、好きなだけ深くできます。制限はありません。たとえば:

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Python console
>>> r.article_set.filter(headline__startswith="This")\n<QuerySet [<Article: This is a test>]>\n\n# Find all Articles for any Reporter whose first name is "John".\n>>> Article.objects.filter(reporter__first_name="John")\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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これは完全一致を意味します:

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Python console
>>> Article.objects.filter(reporter__first_name="John")\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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リレーション先フィールドに2回クエリをかけます。これはWHERE句のAND条件に変換されます:

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Python console
>>> Article.objects.filter(reporter__first_name="John", reporter__last_name="Smith")\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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リレーションのルックアップでは、主キーの値を指定するか、リレーション先オブジェクトを明示的に渡すことができます:

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Python console
>>> Article.objects.filter(reporter__pk=1)\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> Article.objects.filter(reporter=1)\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> Article.objects.filter(reporter=r)\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n\n>>> Article.objects.filter(reporter__in=[1, 2]).distinct()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n>>> Article.objects.filter(reporter__in=[r, r2]).distinct()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n
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インスタンスのリテラルのリストの代わりにクエリセットを使用することもできます:

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Python console
>>> Article.objects.filter(\n...     reporter__in=Reporter.objects.filter(first_name="John")\n... ).distinct()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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逆方向のクエリ:

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Python console
>>> Reporter.objects.filter(article__pk=1)\n<QuerySet [<Reporter: John Smith>]>\n>>> Reporter.objects.filter(article=1)\n<QuerySet [<Reporter: John Smith>]>\n>>> Reporter.objects.filter(article=a)\n<QuerySet [<Reporter: John Smith>]>\n\n>>> Reporter.objects.filter(article__headline__startswith="This")\n<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>\n>>> Reporter.objects.filter(article__headline__startswith="This").distinct()\n<QuerySet [<Reporter: John Smith>]>\n
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逆方向のカウントは distinct() と連動して動作します:

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Python console
>>> Reporter.objects.filter(article__headline__startswith="This").count()\n3\n>>> Reporter.objects.filter(article__headline__startswith="This").distinct().count()\n1\n
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クエリは循環することがあります:

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Python console
>>> Reporter.objects.filter(article__reporter__first_name__startswith="John")\n<QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>\n>>> Reporter.objects.filter(article__reporter__first_name__startswith="John").distinct()\n<QuerySet [<Reporter: John Smith>]>\n>>> Reporter.objects.filter(article__reporter=r).distinct()\n<QuerySet [<Reporter: John Smith>]>\n
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レポーターを削除すると、そのレポーターの記事も削除されます (ForeignKeyが django.db.models.ForeignKey.on_delete に CASCADE をセットして定義されていることが前提です):

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Python console
>>> Article.objects.all()\n<QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>\n>>> Reporter.objects.order_by("first_name")\n<QuerySet [<Reporter: John Smith>, <Reporter: Paul Jones>]>\n>>> r2.delete()\n>>> Article.objects.all()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n>>> Reporter.objects.order_by("first_name")\n<QuerySet [<Reporter: John Smith>]>\n
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クエリで JOIN を使って削除することもできます:

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Python console
>>> Reporter.objects.filter(article__headline__startswith="This").delete()\n>>> Reporter.objects.all()\n<QuerySet []>\n>>> Article.objects.all()\n<QuerySet []>\n
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