Optimizing queries in SerializerMethodField in Django REST framework - django

Question

I am accessing the related field's data in my SerializerMethodField and there is a query for every object that is being rendered. My models look like (will keep short for brevity):
class Listing(models.Model):
variant = models.ForeignKey(to='Variant', related_name='variant_listings')
seller = models.ForeignKey(to='user.Seller', related_name='seller_listings')
locality = models.ForeignKey(to='user.Locality', blank=True, null=True)
price = models.IntegerField(blank=True, null=True)
Variant, Seller and Locality are all related models.
My Viewset:
class ListingViewSet(viewsets.ModelViewSet):
"""Viewset class for Listing model"""
queryset = Listing.objects.all()
serializer_class = ListingSerializer
pagination_class = TbPagination
filter_backends = (filters.DjangoFilterBackend,)
filter_class = ListingFilter
def get_queryset(self):
listing_qs = Listing.objects.filter(status='active')
listing_qs = ListingSerializer.setup_eager_loading(listing_qs)
return listing_qs
And my serializer:
class ListingSerializer(serializers.ModelSerializer):
"""Serializer class for Listing model"""
#staticmethod
def setup_eager_loading(queryset):
queryset = queryset.prefetch_related('variant', 'seller', 'locality')
return queryset
#staticmethod
def get_car_link(obj):
variant_name_slug = obj.variant.name.replace(' ', '-').replace('+', '')
return '/buy-' + obj.seller.city.name.lower() + '/' + variant_name_slug
car_link = serializers.SerializerMethodField(read_only=True)
#staticmethod
def get_car_info(obj):
return {
'id': obj.id,
'variant_name': obj.variant.name,
'localities': obj.locality.name,
}
car_info = serializers.SerializerMethodField(read_only=True)
#staticmethod
def get_image_urls(obj):
caption_ids = [1, 2, 3, 5, 7, 8, 18]
attachments_qs = Attachment.objects.filter(listing_id=obj.id, caption_id__in=caption_ids)
image_urls = []
for attachment in attachments_qs:
url = str(obj.id) + '-' + str(attachment.file_number) + '-360.jpg'
image_urls.append(url)
return image_urls
image_urls = serializers.SerializerMethodField(read_only=True)
class Meta:
model = Listing
fields = ('car_link', 'car_info', 'sort_by', 'image_urls')
For each listing returned by the listing viewset, there is a query for every related field accessed in the SerializerMethodField.
I found some related questions like this. But that didn't help. Also, I tried doing prefetch_related on my get_queryset method of the viewset and also implemented eager loading with the help of this article. But nothing helped.
Is there any way to avoid these queries?
Edit
The get_car_info function written above, contains a few more fields (along with the ones already present) which are required separately in a nested JSON by the name of car_info in the final serialized data that is being rendered at the front end.

Answer 1

I have used this article:
http://ses4j.github.io/2015/11/23/optimizing-slow-django-rest-framework-performance/
I created a set up eager loading method in my serializer, like so:
class EagerGetProjectSerializer(serializers.ModelSerializer):
lead_researcher = UserSerializer()
participating_researcher = UserSerializer(many=True)
client = ProjectClientSerializer()
test_items = TestItemSerializer(many=True)
#staticmethod
def setup_eager_loading(queryset):
queryset = queryset.select_related('lead_researcher', 'client')
queryset = queryset.prefetch_related('participating_researcher',
'test_items')
return queryset
class Meta:
model = Project
fields = '__all__'
Notice that when referencing the objects you want to pull in the serializer you have to use the foreign key related name attribute.
and called in my view, before accessing the serializer:
class SingleProject(APIView):
def get(self, request):
ans = Project.objects.filter(id=project_id)
qs = EagerGetProjectSerializer.setup_eager_loading(ans)
serializer = EagerGetProjectSerializer(qs, many=True)