Django REST Serializer doing N+1 database calls for multiple nested relationship, 3 levels

Question

I have a situation where my model has a Foreign Key relationship:

# models.py
class Child(models.Model):
    parent = models.ForeignKey(Parent,)

class Parent(mo         


        

Answer 1

This question is a bit old but I just came across a very similar problem and managed to reduce the db calls drastically. It seems to me that Django-mptt would make things much easier for you.

One way would be to define a single model with a ForeignKey to. This way you can find out the hierarchy by it's level in the tree. For example:

class Person(MPTTModel):
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children', db_index=True)  

You can find out if the object is a parent by checking if Person.level = 0. If it equals 1, it's a child, 2 grandchild, etc...

Then, you could modify your code to the following:

# serializers.py
class ChildSerializer(serializers.ModelSerializer):
    children = serializers.SerializerMethodField()

    def get_children(self, parent):
        queryset = parent.get_children()
        serialized_data = ChildSerializer(queryset, many=True, read_only=True, context=self.context)
        return serialized_data.data

# views.py
class ParentList(generics.ListAPIView):

    def get_queryset(self):
        queryset = cache_tree_children(Person.objects.all())

With this, you'll eliminate your N+1 problem. If you want to add a new ForeignKey to a genre Model, for example, you could simply modify the last line to:

queryset = cache_tree_children(Person.objects.filter(channel__slug__iexact=channel_slug).select_related('genre'))

Answer 2

As far as I remember, nested serializers have access to prefetched relations, just make sure you don't modify a queryset (i.e. use all()):

class ParentSerializer(serializer.ModelSerializer):
    child = serializers.SerializerMethodField('get_children_ordered')

    def get_children_ordered(self, parent):
        # The all() call should hit the cache
        serialized_data = ChildSerializer(parent.child.all(), many=True, read_only=True, context=self.context)
        return serialized_data.data

    class Meta:
            model = Parent


class ParentList(generics.ListAPIView):

    def get_queryset(self):
        children = Prefetch('child', queryset=Child.objects.select_related('parent'))
        queryset = Parent.objects.prefetch_related(children)
        return queryset

    serializer_class = ParentSerializer
    permission_classes = (permissions.IsAuthenticated,)