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Fixed #25672 -- Clarified why related ManyToManyFields with a custom intermediate model disable the remove() method.

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bphillips 2015-11-25 11:25:21 -05:00 committed by Tim Graham
parent cd40d9e721
commit d508326e2f
1 changed files with 18 additions and 6 deletions
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24
docs/topics/db/models.txt
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@ -513,11 +513,11 @@ the intermediate model::
Unlike normal many-to-many fields, you *can't* use ``add()``, ``create()``, Unlike normal many-to-many fields, you *can't* use ``add()``, ``create()``,
or ``set()`` to create relationships:: or ``set()`` to create relationships::
# THIS WILL NOT WORK >>> # THIS WILL NOT WORK
>>> beatles.members.add(john) >>> beatles.members.add(john)
# NEITHER WILL THIS >>> # NEITHER WILL THIS
>>> beatles.members.create(name="George Harrison") >>> beatles.members.create(name="George Harrison")
# AND NEITHER WILL THIS >>> # AND NEITHER WILL THIS
>>> beatles.members.set([john, paul, ringo, george]) >>> beatles.members.set([john, paul, ringo, george])
Why? You can't just create a relationship between a ``Person`` and a ``Group`` Why? You can't just create a relationship between a ``Person`` and a ``Group``
@ -529,9 +529,21 @@ The only way to create this type of relationship is to create instances of the
intermediate model. intermediate model.
The :meth:`~django.db.models.fields.related.RelatedManager.remove` method is The :meth:`~django.db.models.fields.related.RelatedManager.remove` method is
disabled for similar reasons. However, the disabled for similar reasons. For example, if the custom through table defined
:meth:`~django.db.models.fields.related.RelatedManager.clear` method can be by the intermediate model does not enforce uniqueness on the
used to remove all many-to-many relationships for an instance:: ``(model1, model2)`` pair, a ``remove()`` call would not provide enough
information as to which intermediate model instance should be deleted::
>>> Membership.objects.create(person=ringo, group=beatles,
... date_joined=date(1968, 9, 4),
... invite_reason="You've been gone for a month and we miss you.")
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>, <Person: Paul McCartney>, <Person: Ringo Starr>]>
>>> # THIS WILL NOT WORK BECAUSE IT CANNOT TELL WHICH MEMBERSHIP TO REMOVE
>>> beatles.members.remove(ringo)
However, the :meth:`~django.db.models.fields.related.RelatedManager.clear`
method can be used to remove all many-to-many relationships for an instance::
>>> # Beatles have broken up >>> # Beatles have broken up
>>> beatles.members.clear() >>> beatles.members.clear()