save() method called twice when using post_generation

I just realised that my model factory was calling the model save method twice when defining post_generation function. Here is the many-to-many example:

class UserFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = models.User

    name = "John Doe"

    @factory.post_generation
    def groups(self, create, extracted, **kwargs):
        if not create:
            # Simple build, do nothing.
            return

        if extracted:
            # A list of groups were passed in, use them
            for group in extracted:
                self.groups.add(group)

Calling UserFactory triggers save() on User model twice.

This happens because: 1/ The DjangoModelFactory redefines _after_postgeneration:

    @classmethod
    def _after_postgeneration(cls, obj, create, results=None):
        """Save again the instance if creating and at least one hook ran."""
        if create and results:
            # Some post-generation hooks ran, and may have modified us.
            obj.save()

2/ This results comes from the call to our post_generation function, which with previous User example returns {'groups': None}

Which use case has led to this post-generation saving? If some data had to be saved in a custom post_generation function, nothing is preventing to explicitly call save on obj.

Do I have missed something? Should I override this method if I don’t want to get this double save() on my model, or there is a better way?

[rbarrois]

This is useful for declarations such as:

class UserFactory(factory.Factory):
    class Meta:
        model = User

    username = factory.Faker('username')
    password = factory.PostGenerationMethodCall('set_password', 'Password123')

In such cases, we want to have the factory be saved again after the call to set_password.

What kinds of problems are caused by this double save()?

[grutts]

I had this problem as well, spotted it when one of my tests was behaving unexpectedly. I overrode _after_postgeneration to do nothing