How to prefetch_related() for GenericForeignKeys? - django

Question

I have a List that consists of ListItems. These ListItems then point towards either a ParentItem or a ChildItem model via a GenericForeignKey:
# models.py
class List(models.Model):
title = models.CharField()
class ListItem(models.Model):
list = models.ForeignKey(List, related_name="list_items")
order = models.PositiveSmallIntegerField()
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey("content_type", "object_id")
class ParentItem(models.Model):
title = models.CharField()
class ChildItem(models.Model):
title = models.CharField()
parent = models.ForeignKey(ParentItem, related_name="child")
I want to display a list of all my Lists with their ListItems and respective ItemA/ItemB data using ListSerializer:
# serializers.py
class ParentItemSerializer(serializers.ModelSerializer):
class Meta:
model = ParentItem
fields = ["title"]
class ChildItemSerializer(serializers.ModelSerializer):
parent = ParentItemSerializer()
class Meta:
model = ChildItem
fields = ["title", "parent"]
class ListItemSerializer(serializers.ModelSerializer):
contents = serializers.SerializerMethodField()
class Meta:
model = ListItem
fields = ["contents"]
def get_contents(self, obj):
item = obj.content_object
type = item.__class__.__name__
if type == "ParentItem":
return ParentItemSerializer(item).data
elif type == "ChildItem":
return ChildItemSerializer(item).data
class ListSerializer(serializers.ModelSerializer):
items = serializers.SerializerMethodField()
class Meta:
model = List
fields = ["title", "items"]
def get_items(self, obj):
return ListItemSerializer(obj.list_items, many=True).data
How can I optimize my List queryset to prefetch these GenericForeignKey relationships?
# views.py
class ListViewSet(viewset.ModelViewSet):
queryset = List.objects.all()
serializer_class = ListSerializer
List.objects.all().prefetch_related("list_items") works but the following does not seem to work:
List.objects.all().prefetch_related(
"list_items",
"list_items__content_object",
"list_items__content_object__parent",
)
I've read the documentation on prefetch_related which suggests it should work:
While prefetch_related supports prefetching GenericForeignKey
relationships, the number of queries will depend on the data. Since a
GenericForeignKey can reference data in multiple tables, one query per
table referenced is needed, rather than one query for all the items.
There could be additional queries on the ContentType table if the
relevant rows have not already been fetched.
but I don't know if that's applicable to DRF.
Edit: Some better success when I move some of the prefetching to the serializer:
class ListSerializer(serializers.ModelSerializer):
def get_items(self, obj):
return ListItemSerializer(obj.list_items.all().prefetch_related("content_object"), many=True).data

Answer 1

As per my edit, prefetching works as intended when I move the relevant fields when they're queried in the serializer instead of cramming it all in the view's queryset.