Parse GraphQL query to find fields to be able to prefetch_related? - django

Question

When using DjangoListObjectType from graphene_django_extras, I can define a custom qs property on a SearchFilter.
The qs function has the object as its only argument, and through that I can get the request, and in turn the query string which includes the queried fields.
Before hacking together my own parser to get these fields, am I going about this the wrong way? Or is there something else out there?
The idea is to have quite a rigid approach, as we have 7 types of paginated list types with fields that result in a few unnecessary database hits, so we want to prefetch a few fields.
Graphene has a dataloader approach which kind of looks right, but more complicated than just prefetching them at the qs stage.
Re the dataloader approach, I tried something like this
class UserLoader(DataLoader):
def batch_load_fn(self, keys):
users = {user.id: user for user in User.objects.filter(id__in=keys)}
return Promise.resolve([users.get(user_id) for user_id in keys])
user_loader = UserLoader()
class BookType(DjangoObjectType):
...
author = graphene.Field(UserType)
def resolve_author(self, *args, **kwargs):
return user_loader.load(self.author.id)
Which kind of obviously feels completely wrong, but is what they do in the docs.

Answer 1

Using a DataLoader is the correct approach but do self.author_id instead of self.author.id.
By calling self.author.id you're fetching the author for each book (one SQL query per author) then getting the id attribute from each author.
When you add the author ForeignKey to your Book model, behind the scenes, Django adds a column to the books table—author_id. You can pass this value to your DataLoader and have it fetch all the corresponding Authors in a single SQL query using a SELECT IN.
https://docs.djangoproject.com/en/3.2/ref/models/fields/#database-representation
You'll notice that the Graphene Docs are doing the same thing with best_friend_id in their example.