Support select_related and prefetch_related for inherited models and custom queryset for child models

[k4anubhav]

add select_polymorphic_related, prefetch_polymorphic_related and custom_queryset methods for select and prefetch on inherited models

Triggers Django errors when that field doesn't exist in a particular subclass

Example works exactly same as django select_related and prefetch_related

queryset = Project.objects.select_polymorphic_related(
    ArtProject, 'artist', 'canvas__painter'
).select_polymorphic_related(
    ResearchProject, 'supervisor',
)


queryset = Project.objects.prefetch_polymorphic_related(
    ArtProject, 'artist', Prefetch('canvas', queryset=Project.objects.annotate(size=F('width') * F('height')))
).prefetch_polymorphic_related(
    ResearchProject, 'authors',
)


queryset = Project.objects.custom_queryset(ArtProject, ArtProject._base_objects.annotate(cost=F('cost') + F('canvas_cost'))))

[k4anubhav]

#198

[0legRadchenko]

Hi! Thank you for your commits! =)
Right now there are some problems If Im trying to combine select_related and select_polymorphic_related in one query (in such case It dupicates queries). So its not possible to use them simultaneously without making extra duplicated queries.

But if I use select_polymorphic_related without select_related everything working fine and vice versa If I use select_related without select_polymorphic_related in one query everything working good as well.

I've sent you additional info in telegram :)

[0legRadchenko]

Now all is great 🙌🏻 Ive tested select_related and select_prefetch_related together and also separatly and did not manage to find any duplicates in queries and so far so good.

[Safrone]

Anything holding this back? Feels like it would be a big win if it "just works"

[alphatownsman]

it would be really really nice to have this

[mazulo]

Hey team, when can we expect to get this merged?

[FrancoisDupayrat]

Thanks a lot for this PR. It single-handedly solved the biggest performance issue in my production app.
I can confirm it works. Hopefully it will get merged.

[Safrone]

I created a Gist showing how to use this enhancement without having to pin this PR version or modify Django Polymorphic:
https://gist.github.com/Safrone/7c24b492a791679e6e94cc765459dc78

I haven't gotten to do extensive testing on it so anyone using it finding issues please let me know