{"title":"Många-till-en-relationer","version":"6.0","locale":"sv","docname":"topics/db/examples/many_to_one","url":"/sv/6.0/topics/db/examples/many_to_one/","canonical":"https://djangodocs.dev/sv/6.0/topics/db/examples/many_to_one/","summary":"För att definiera en många-till-en-relation, använd ForeignKey . I detta exempel kan en Reporter associeras med många Article -objekt, men en Article kan bara ha…","html":"

Många-till-en-relationerLink to this heading#

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För att definiera en många-till-en-relation, använd ForeignKey.

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I detta exempel kan en Reporter associeras med många Article-objekt, men en Article kan bara ha ett Reporter-objekt:

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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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Nedan följer exempel på operationer som kan utföras med hjälp av Python API-faciliteterna.

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Skapa några reportrar:

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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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Skapa en artikel:

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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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Observera att du måste spara ett objekt innan det kan tilldelas en relation med främmande nyckel. Om du t.ex. skapar en Article med en osparad Reporter uppstår ValueError:

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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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Artikelobjekt har tillgång till sina relaterade Reporter-objekt:

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Python console
>>> r = a.reporter\n
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Skapa en artikel via Reporter-objektet:

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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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Skapa en ny artikel:

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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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Lägg till samma artikel i en annan artikeluppsättning - kontrollera att den flyttas:

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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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Att lägga till ett objekt av fel typ ger upphov till 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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Observera att i det sista exemplet har artikeln flyttats från John till Paul.

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Relaterade chefer stöder även fältuppslagningar. API:et följer automatiskt relationer så långt som du behöver. Använd dubbla understrykningar för att separera relationer. Detta fungerar så många nivåer djupt som du vill. Det finns ingen begränsning. Till exempel:

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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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Exakt matchning är underförstått här:

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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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Fråga två gånger över det relaterade fältet. Detta översätts till ett AND-villkor i WHERE-klausulen:

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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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För den relaterade uppslagningen kan du ange ett primärnyckelvärde eller skicka det relaterade objektet explicit:

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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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Du kan också använda en queryset i stället för en bokstavlig lista med instanser:

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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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Förfrågan i motsatt riktning:

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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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Räkning i motsatt riktning fungerar i kombination med 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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Frågor kan gå runt i cirklar:

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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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Om du tar bort en reporter kommer deras artiklar att raderas (förutsatt att ForeignKey definierades med django.db.models.ForeignKey.on_delete inställd på CASCADE, vilket är standard):

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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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Du kan ta bort med hjälp av en JOIN i frågan:

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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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