{"title":"Många-till-många-relationer","version":"6.0","locale":"sv","docname":"topics/db/examples/many_to_many","url":"/sv/6.0/topics/db/examples/many_to_many/","canonical":"https://djangodocs.dev/sv/6.0/topics/db/examples/many_to_many/","summary":"Om du vill definiera en relation mellan många och många använder du ManyToManyField.","html":"

Många-till-många-relationerLink to this heading#

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Om du vill definiera en relation mellan många och många använder du ManyToManyField.

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I det här exemplet kan en ”artikel” publiceras i flera ”publikationsobjekt” och en ”publikation” har flera ”artikelobjekt”:

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Python
from django.db import models\n\n\nclass Publication(models.Model):\n    title = models.CharField(max_length=30)\n\n    class Meta:\n        ordering = ["title"]\n\n    def __str__(self):\n        return self.title\n\n\nclass Article(models.Model):\n    headline = models.CharField(max_length=100)\n    publications = models.ManyToManyField(Publication)\n\n    class Meta:\n        ordering = ["headline"]\n\n    def __str__(self):\n        return self.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 instanser av typen Publication:

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Python console
>>> p1 = Publication(title="The Python Journal")\n>>> p1.save()\n>>> p2 = Publication(title="Science News")\n>>> p2.save()\n>>> p3 = Publication(title="Science Weekly")\n>>> p3.save()\n
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Skapa en Artikel:

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Python console
>>> a1 = Article(headline="Django lets you build web apps easily")\n
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Du kan inte associera den med en Publikation förrän den har sparats:

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Python console
>>> a1.publications.add(p1)\nTraceback (most recent call last):\n...\nValueError: "<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.\n
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Spara det!

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Python console
>>> a1.save()\n
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Koppla Artikeln till en Publikation:

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Python console
>>> a1.publications.add(p1)\n
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Skapa en ny ”artikel” och ställ in den så att den visas i dess publikationer:

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Python console
>>> a2 = Article(headline="NASA uses Python")\n>>> a2.save()\n>>> a2.publications.add(p1, p2)\n>>> a2.publications.add(p3)\n
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Att lägga till en andra gång är OK, det kommer inte att duplicera relationen:

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Python console
>>> a2.publications.add(p3)\n
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Att lägga till ett objekt av fel typ ger upphov till TypeError:

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Python console
>>> a2.publications.add(a1)\nTraceback (most recent call last):\n...\nTypeError: 'Publication' instance expected\n
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Skapa och lägg till en Publikation till en Artikel i ett steg med create():

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Python console
>>> new_publication = a2.publications.create(title="Highlights for Children")\n
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objekten Article har tillgång till sina relaterade objekt Publication:

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Python console
>>> a1.publications.all()\n<QuerySet [<Publication: The Python Journal>]>\n>>> a2.publications.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n
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objekt av typen Publication har tillgång till sina relaterade objekt av typen Article:

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Python console
>>> p2.article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n>>> p1.article_set.all()\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Publication.objects.get(id=4).article_set.all()\n<QuerySet [<Article: NASA uses Python>]>\n
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Många-till-många-relationer kan efterfrågas med hjälp av lookups across relationships:

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Python console
>>> Article.objects.filter(publications__id=1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications__pk=1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications=1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications=p1)\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n\n>>> Article.objects.filter(publications__title__startswith="Science")\n<QuerySet [<Article: NASA uses Python>, <Article: NASA uses Python>]>\n\n>>> Article.objects.filter(publications__title__startswith="Science").distinct()\n<QuerySet [<Article: NASA uses Python>]>\n
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Funktionen count() respekterar även distinct():

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Python console
>>> Article.objects.filter(publications__title__startswith="Science").count()\n2\n\n>>> Article.objects.filter(publications__title__startswith="Science").distinct().count()\n1\n\n>>> Article.objects.filter(publications__in=[1, 2]).distinct()\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n>>> Article.objects.filter(publications__in=[p1, p2]).distinct()\n<QuerySet [<Article: Django lets you build web apps easily>, <Article: NASA uses Python>]>\n
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Omvända m2m-frågor stöds (dvs. börjar vid den tabell som inte har en ManyToManyField):

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Python console
>>> Publication.objects.filter(id=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(pk=1)\n<QuerySet [<Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__headline__startswith="NASA")\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__id=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article__pk=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article=1)\n<QuerySet [<Publication: The Python Journal>]>\n>>> Publication.objects.filter(article=a1)\n<QuerySet [<Publication: The Python Journal>]>\n\n>>> Publication.objects.filter(article__in=[1, 2]).distinct()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n>>> Publication.objects.filter(article__in=[a1, a2]).distinct()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]>\n
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Att utesluta ett relaterat objekt fungerar också som du förväntar dig (även om den SQL som används är lite komplicerad):

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Python console
>>> Article.objects.exclude(publications=p2)\n<QuerySet [<Article: Django lets you build web apps easily>]>\n
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Om vi tar bort en Publication kommer dess relaterade Article-instanser inte att kunna komma åt den:

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Python console
>>> p1.delete()\n>>> Publication.objects.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]>\n>>> a1 = Article.objects.get(pk=1)\n>>> a1.publications.all()\n<QuerySet []>\n
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Om vi tar bort en ”artikel” kommer dess relaterade ”publikationsinstanser” inte att kunna komma åt den:

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Python console
>>> a2.delete()\n>>> Article.objects.all()\n<QuerySet [<Article: Django lets you build web apps easily>]>\n>>> p2.article_set.all()\n<QuerySet []>\n
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Lägga till via den ”andra” änden av en m2m:

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Python console
>>> a4 = Article(headline="NASA finds intelligent life on Earth")\n>>> a4.save()\n>>> p2.article_set.add(a4)\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>]>\n>>> a4.publications.all()\n<QuerySet [<Publication: Science News>]>\n
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Lägga till via den andra änden med hjälp av nyckelord:

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Python console
>>> new_article = p2.article_set.create(headline="Oxygen-free diet works wonders")\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>\n>>> a5 = p2.article_set.all()[1]\n>>> a5.publications.all()\n<QuerySet [<Publication: Science News>]>\n
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Ta bort Publication från en Article:

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Python console
>>> a4.publications.remove(p2)\n>>> p2.article_set.all()\n<QuerySet [<Article: Oxygen-free diet works wonders>]>\n>>> a4.publications.all()\n<QuerySet []>\n
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Och från den andra änden:

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Python console
>>> p2.article_set.remove(a5)\n>>> p2.article_set.all()\n<QuerySet []>\n>>> a5.publications.all()\n<QuerySet []>\n
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Relationssatser kan ställas in:

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Python console
>>> a4.publications.all()\n<QuerySet [<Publication: Science News>]>\n>>> a4.publications.set([p3])\n>>> a4.publications.all()\n<QuerySet [<Publication: Science Weekly>]>\n
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Relationssatser kan rensas:

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Python console
>>> p2.article_set.clear()\n>>> p2.article_set.all()\n<QuerySet []>\n
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Och du kan rensa från andra änden:

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Python console
>>> p2.article_set.add(a4, a5)\n>>> p2.article_set.all()\n<QuerySet [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]>\n>>> a4.publications.all()\n<QuerySet [<Publication: Science News>, <Publication: Science Weekly>]>\n>>> a4.publications.clear()\n>>> a4.publications.all()\n<QuerySet []>\n>>> p2.article_set.all()\n<QuerySet [<Article: Oxygen-free diet works wonders>]>\n
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Återskapa den Artikel och Publikation som vi har tagit bort:

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Python console
>>> p1 = Publication(title="The Python Journal")\n>>> p1.save()\n>>> a2 = Article(headline="NASA uses Python")\n>>> a2.save()\n>>> a2.publications.add(p1, p2, p3)\n
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Ta bort några Publication-instanser och referenserna till de borttagna publikationerna kommer inte längre att ingå i de relaterade posterna:

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Python console
>>> Publication.objects.filter(title__startswith="Science").delete()\n>>> Publication.objects.all()\n<QuerySet [<Publication: Highlights for Children>, <Publication: The Python Journal>]>\n>>> Article.objects.all()\n<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>]>\n>>> a2.publications.all()\n<QuerySet [<Publication: The Python Journal>]>\n
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Bulkradera vissa artiklar - referenser till raderade objekt bör tas bort:

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Python console
>>> q = Article.objects.filter(headline__startswith="Django")\n>>> print(q)\n<QuerySet [<Article: Django lets you build web apps easily>]>\n>>> q.delete()\n
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Efter delete() måste QuerySet-cachen rensas och de refererade objekten bör försvinna:

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