factory_boy: passing model instances for a RelatedFactory

Question

I figured instead of passing data to a factory's RelatedFactory in the form of ATTR__SUBATTR, you should also be able to just directly pass already existing instances. Unless I'm missing something very obvious, this just doesn't seem to work. Have a look:

class Owner(models.Model):
    name = models.CharField()

class Item(models.Model):
    name = models.CharField()
    owner = models.ForeignKey(Owner, null = True, related_name = 'items')

class ItemFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = Item

class OwnerFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = Owner
    items = factory.RelatedFactory(ItemFactory, 'owner')

item = Item.objects.create(name='Foo')
alice = OwnerFactory(name='Alice', items__name='Bar')
alice.items.all()
<QuerySet [<Item: Bar>]>
bob = OwnerFactory(name='Bob', items=item) # or items = [item] doesn't matter
bob.items.all()
<QuerySet []>

Been working on making my factories nice and DRY and hit this roadblock. Wrote my own adaption of RelatedFactory that allows for multiple values to be handled at once, which works fine if you are creating new objects in the process - but not if you are using already existing ones.

Example that works: OwnerFactory(items__name=['Foo','Bar'])=> Foo and Bar in owner.items.
Example that does not work: OwnerFactory(items=[foo,bar])=> owner.items is empty
Note that I have used the default RelatedFactory in the big example at the top.

I have been all over the documentation for factory_boy the entire day, but couldn't find a solution and tunnel vision has taken over now, prohibiting any new insight.

Answer 1

You are looking for http://factoryboy.readthedocs.io/en/latest/recipes.html#simple-many-to-many-relationship

class ItemFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = Item

class OwnerFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = Owner

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

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