How to define custom polymorphic QuerySets and Managers?

[fed239]

I'd like to add filter_by_user methods to all QuerySet classes defined for child polymorphic models and use them to filter objects of every model when called for the base model. filter_by_user methods may be different for different child models. Is this possible?

I tried like in the following code but it didn't work.

class MyBaseQuerySet(PolymorphicQuerySet):
    def filter_by_user(self, user):
        return self.all()


class MyBaseModel(PolymorphicModel):
    ...
    objects = PolymorphicManager(MyBaseQuerySet)


class MyChild1QuerySet(MyBaseQuerySet):
    def filter_by_user(self, user):
        return self.filter(fieldA__lt=user.profile.propA)


class MyChild1Model(MyBaseModel):
    fieldA = models.IntegerField()
    ...
    objects = PolymorphicManager(MyChild1QuerySet)


class MyChild2QuerySet(MyBaseQuerySet):
    def filter_by_user(self, user):
        return self.filter(fieldB__gt=user.profile.propB)


class MyChild2Model(MyBaseModel):
    fieldB = models.IntegerField()
    ...
    objects = PolymorphicManager(MyChild2QuerySet)

When I call MyBaseModel.objects.filter_by_user(john) magic doesn't happen and only base queryset filter_by_user is called.

[dsummersl]

I think I'm seeing the same thing for this scenario:

class MyBaseModel(PolymorphicModel):
    ...
    # no "objects =" defined.

class SubclassFeedManager(PolymorphicManager):
    def get_queryset(self):
        ...

class MySubModel(MyBaseModel):
    objects = SubclassFeedManager()


...

MySubModel.objects.all()

This call doesn't use the SubclassFeedManager() at all.

[MastaChimp]

Can somebody please suggest a workaround for this issue? I also want the custom manager to be called on each of the child classes when you call the base class custom manager.

[bckohan]

Hello @fed239 thanks for your question - sorry its been a decade for someone to get back to you!

There are a few concepts to understand here. First - the difference between a manager and a queryset. Model.objects is a Manager instance. Methods on that manager instance may return QuerySet instances (e.g. all). Which brings me to the first point:

• If you write a custom QuerySet - the methods you add to that QuerySet will not be available on the manager by default unless you create a manager class from the QuerySet.

There are several ways to specify custom managers and querysets:

1. PolymorphicManager.from_queryset()

    class CustomQuerySet(PolymorphicQueryset):
        ....
    class Model(PolymorphicModel):
        objects = PolymorphicManager.from_queryset(CustomQuerySet)

2. CustomQuerySet.as_manager()

    class CustomQuerySet(PolymorphicQueryset):
        ....
    class Model(PolymorphicModel):
        objects = CustomQuerySet.as_manager()

3. Extend a custom manager class from the result of from_queryset:

    class CustomQuerySet(PolymorphicQueryset):
        ....
    class CustomManager(PolymorphicManager.from_queryset(CustomQuerySet)):
        ....
    class Model(PolymorphicModel):
        objects = CustomManager()

1 and 2 are functionally equivalent - as_manager() actually calls PolymorphicManager.from_queryset() under the covers. The only difference is that as_manager() will extend from PolymorphicManager by default unless you override the function in your QuerySet implementation. from_queryset() gives you the chance to combine any arbitrary subclass of PolymorphicManager with any arbitrary subclass of PolymorphicQueryset. You might use pattern 3 if you want to define functions on your manager that would not be available on a queryset returned by the manager (e.g. special versions of create()).

Applying this to your code you could write:

class MyBaseQuerySet(PolymorphicQuerySet):
    def filter_by_user(self, user):
        return self.all()


class MyBaseModel(PolymorphicModel):
    ...
    objects = MyBaseQuerySet.as_manager()


class MyChild1QuerySet(MyBaseQuerySet):
    def filter_by_user(self, user):
        return self.filter(fieldA__lt=user.profile.propA)


class MyChild1Model(MyBaseModel):
    fieldA = models.IntegerField()
    ...
    objects = MyChild1QuerySet.as_manager()


class MyChild2QuerySet(MyBaseQuerySet):
    def filter_by_user(self, user):
        return self.filter(fieldB__gt=user.profile.propB)


class MyChild2Model(MyBaseModel):
    fieldB = models.IntegerField()
    ...
    objects = MyChild2QuerySet.as_manager()

@dsummersl your example code does work. I tested it against the current release - it looks like a PR fixing this problem was added in 2013: #33

[fed239]

Hi @bckohan! Appreciate that you came back to this, the explanation makes sense. This discussion also brought back some good memories from my Django days, which were over like 5 years ago 🤠

[dsummersl]

thx @bckohan!