{"title":"Many-to-one relationships","version":"2.1","locale":"ko","docname":"topics/db/examples/many_to_one","url":"/ko/2.1/topics/db/examples/many_to_one/","canonical":"https://djangodocs.dev/ko/2.1/topics/db/examples/many_to_one/","summary":"To define a many-to-one relationship, use ForeignKey : Code Copy from django.db import models class Reporter ( models . Model ): first_name = models . CharField (…","html":"

Many-to-one relationshipsLink to this heading#

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To define a many-to-one relationship, use ForeignKey:

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Code
from django.db import models\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 "%s %s" % (self.first_name, self.last_name)\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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What follows are examples of operations that can be performed using the Python\nAPI facilities.

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Create a few Reporters:

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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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Create an Article:

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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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Note that you must save an object before it can be assigned to a foreign key\nrelationship. For example, creating an Article with unsaved Reporter\nraises ValueError:

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Python console
>>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')\n>>> Article.objects.create(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)\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 objects have access to their related Reporter objects:

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Python console
>>> r = a.reporter\n
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Create an Article via the Reporter object:

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Python console
>>> new_article = r.article_set.create(headline="John's second story", pub_date=date(2005, 7, 29))\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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Create a new article:

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Python console
>>> new_article2 = Article.objects.create(headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r)\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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Add the same article to a different article set - check that it moves:

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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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Adding an object of the wrong type raises TypeError:

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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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Note that in the last example the article has moved from John to Paul.

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Related managers support field lookups as well.\nThe API automatically follows relationships as far as you need.\nUse double underscores to separate relationships.\nThis works as many levels deep as you want. There’s no limit. For example:

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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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Exact match is implied here:

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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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Query twice over the related field. This translates to an AND condition in the\nWHERE clause:

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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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For the related lookup you can supply a primary key value or pass the related\nobject explicitly:

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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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You can also use a queryset instead of a literal list of instances:

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Python console
>>> Article.objects.filter(reporter__in=Reporter.objects.filter(first_name='John')).distinct()\n<QuerySet [<Article: John's second story>, <Article: This is a test>]>\n
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Querying in the opposite direction:

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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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Counting in the opposite direction works in conjunction with distinct():

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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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Queries can go round in circles:

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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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If you delete a reporter, his articles will be deleted (assuming that the\nForeignKey was defined with django.db.models.ForeignKey.on_delete set to\nCASCADE, which is the default):

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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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You can delete using a JOIN in the query:

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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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