django使用mptt和polymorphic进行模型化

Question

I have a PolymorphicMPTTModel and models are:

class AbstractCategory(PolymorphicMPTTModel):
    name = models.CharField(max_length=100)
    parent = PolymorphicTreeForeignKey('self', null=True, blank=True,
                                       related_name='sub_categories')
class Category(AbstractCategory):
    def get_fields(self):
        pass 

class FakeCategory(AbstractCategory):
    def get_fields(self):
        pass
class AbstractProductModel(PolymorphicModel):
    name = models.CharField(max_length=100)

class ProductModel(AbstractProductModel):
    category = models.ForeignKey('category.Category', related_name='product_models')

class FakeProductModel(AbstractProductModel):
    category = models.ForeignKey('category.AbstractCategory', related_name='fake_product_models')

Assume i have 3 objects of Category : Digital, Mobile-->Digital, Camera-->Digital. Digital is parent of Mobile and Camera and this is simple example of tree structure. I want to get all models that their category is Digital or sub category of Digital like Mobile or Camera. for that i'm using:

category = Category.objects.get(name='Digital')
descendants = category.get_descendants(include_self=True)
models = ProductModel.objects.filter(category__in=descendants).all()

I get this error: Cannot resolve keyword 'abstractcategory_ptr' into field. Choices are: category, ...

and can't find the reason.

Answer 1

While I don't quite understand why the category__in filter breaks (it should work!), you can implement this query in a much better way. When you join using the mptt fields, everything happens in a single query instead of a WHERE x IN (SELECT ..) subquery.

class ProductModelQuerySet(PolymorphicQuerySet):
    def get_descendants(self, category, include_self=False):
        # narrow one level deeper for include_self=False effect
        narrow = 0 if include_self else 1

        # MPTT optimization, no need for a subquery.
        return self.filter(
            category__tree_id=category.tree_id,
            category__lft__gte=category.lft + narrow,
            category__rght__lte=category.rght - narrow,
        )

and you can query that using:

models = ProductModel.objects.get_descendants(category)