CODEWITHSUNDEEP
pythondjango
Code-Apprentice
Code-Apprentice

Reputation: 83557

Disable unique constraint on Django's ManyToManyField

From the Django documentation for ManyToManyField:

If you don’t want multiple associations between the same instances, add a UniqueConstraint including the from and to fields. Django’s automatically generated many-to-many tables include such a constraint.

How do I disable this constraint? Is the only way to provide an explicit through table? Or is there a way to tell Django to not add the UniqueConstraint to the generated many-to-many-table?

Upvotes: 1

Views: 1125

Answers (2)

user1600649
user1600649

Reputation:

You can monkey patch django.db.models.fields.related.create_many_to_many_intermediary_model, which creates the intermediary model and that's the closest you can get to it. It is called from the contribute_to_class method on ManyToManyField. Of course, this is more work than specifying a through model, but if you have a use case where all (or most) of your intermediaries require no constraint, then this is worth looking into.

I don't see a supported way to override just this constraint.

And yes, you can subclass ManyToManyField and set the through model there through a utility function that is identical to the above mentioned, barring the constraint and then call super(). This would evade the constraint, because contribute_to_class doesn't call the above method when a through model is already set:

        if not cls._meta.abstract:
            if self.remote_field.through:
                def resolve_through_model(_, model, field):
                    field.remote_field.through = model
                lazy_related_operation(resolve_through_model, cls, self.remote_field.through, field=self)
            elif not cls._meta.swapped:
                self.remote_field.through = create_many_to_many_intermediary_model(self, cls)

Both methods have the same caveat: you need to manually synchronize your alternate utility with Django's upon Django upgrades, all for the purpose of removing one line: 'unique_together': (from_, to),.

Upvotes: 1

Jonatan
Jonatan

Reputation: 1242

m2m tables are linked by an intermediary table which manages the relationships and has a unique_together constraint. This table is automatically created but you can use your own table as the doc says with the through argument. You can read about it in the docs. In your case you need disable the unique_together constraint by defining your own intermediary table.

Other options are:

  1. Extend the default ManyToManyField and override the contribute_to_class method.
  2. Edit the default ManyToManyField directly as @Melvyn mentioned.

Upvotes: 2

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