graphene-django: DjangoConnectionField.resolve_connection replaces resolved queryset with clone, wiping query cache (causing N+1)

I’m new to graphene-django so please bear with me if my issue is nonsensical.

I’ve been spending some time trying to figure out why, when using a resolution method in combination with a DjangoFilterConnectionField any prefetches are being unused. I’ve traced it down to a few things.

Within DjangoFilterConnectionField.merge_querysets an & operator is used to merge two Django QuerySets. The __and__ method on QuerySet creates a new instance of its class, thereby losing any internal caching that may have been accomplished when using prefetch_related(). There doesn’t seem to be much of a way around this without accessing private attributes of the QuerySet object, which is clearly bad practice.

I then noticed that DjangoConnectionField.resolve_connection checks to see if iterable is not default_manager. If it’s not (which as far as I can tell it never will be if you have a resolve_things method on your Query class) it will then follow down the aforementioned path which winds up wiping cache because it creates a new QuerySet.

I wonder, what is the purpose of the following code in resolve_connection? If I disable it, the queryset my resolve_things method returns is actually used and thus the prefetch works.

            if iterable is not default_manager:
                default_queryset = maybe_queryset(default_manager)
                iterable = cls.merge_querysets(default_queryset, iterable)

For reference, my implementation is as follows:

class PostType(DjangoObjectType):
    class Meta:
        model = Post
        filter_fields = ['id', 'title']
        interfaces = (graphene.relay.Node, )


class Query(graphene.ObjectType):
    posts = DjangoFilterConnectionField(PostType)
    
    def resolve_posts(self, info, **kwargs):
        return Post.objects.prefetch_related('customer_posts').all()

About this issue

Original URL
State: closed
Created 6 years ago
Reactions: 2
Comments: 15 (2 by maintainers)

Most upvoted comments

merge_queryset is also impacting #758 & #787

I don’t see an easy way to fix this. Maybe instead of trying to union querysets we pass through a single queryset.

zbyte64 on Oct 2, 2019

We ended up overriding the method like this:

class PrefetchingConnectionField(DjangoConnectionField):

    @classmethod
    def merge_querysets(cls, default_queryset, queryset):
        return queryset

Now in your model you redefine the field and implement resolve.

So far it works, but I would love to know what am I breaking with this:

# Child has a FK to Parent and the reverse label is "children"
class ParentNode(DjangoObjectType, CommonInteractionNode):
    ...
    children = PrefetchingConnectionField(ChildNode)

    class Meta:
        model = Parent
        interfaces = (Node,)
        filter_fields = []

    def resolve_children(self, info):
        return self.children.all()

# Parent has a FK to Root and you want to prefetch in resolve_parents
class RootNode(DjangoObjectType):
    class Meta:
        model = Root
        interfaces = (Node,)
        filter_fields = []
        
    def resolve_parents(self, info):
        return Parents.objects.filter(root__id=self.id).select_related(
            "some_fk",
        ).prefetch_related(
            "children"
       )

dani0805 on Aug 31, 2018