Speeding up Django Rest Framework Model Serializer N+1 Query problem

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I have a DRF ModelSerializer class that serializes anOrder model. This serializer has a field:

num_modelA = serializers.SerializerMethodField() `

def get_num_modelA(self, o):
    r = ModelA.objects.filter(modelB__modelC__order=o).count()

    return r

Where ModelA has a ForeignKey field modelB, ModelB has a ForeignKey field modelC, and ModelC has a ForeignKey field order.

The problem with this is obviously that for each order that gets serialized it makes an additional query to the DB which slows performance down.

I've implemented a static method setup_eager_loading as described here that fixed the N+1 query problem for other fields I was having.

@staticmethod
def setup_eager_loading(queryset):
    # select_related for "to-one" relationships
    queryset = queryset.select_related('modelD','modelE')
    
    return queryset

My idea was I could use prefetch_related as well to reduce the number of queries. But I am unsure how to do this since Order and ModelA are separated by multiple foreign keys. Let me know if any other information would be useful

django-rest-framework serialization api django
1 Answers

You can work with an annotation:

from django.db.models import Count

# …

@staticmethod
def setup_eager_loading(queryset):
    # select_related for "to-one" relationships
    return queryset.select_related('modelD','modelE').annotate(
        num_modelA=Count('modelC__modelB__modelA')
    )

in the serializer for your Order, you can then use num_modelA as an IntegerField:

from rest_framework import serializers

class OrderSerializer(serializers.ModelSerializer):
    num_modelA = serializers.IntegerField()

    class Meta:
        model = Order
        fields = ['num_modelA', 'and', 'other', 'fields']
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