Django rest framework ListApiView slow query

December 24, 2021

I have a displays table with 147 rows. I am not doing any heavy computation on this data set I just need to get it fast from the database. For now, the load time is 3-4 seconds. Other data comes really fast, why? Does the ListApiView work slow?

@permission_classes([AllowAny])
class DisplaysList(generics.ListAPIView):
    queryset = Displays.objects.all()
    serializer_class = serializers.DisplaySerializer




class Displays(models.Model):
    name = models.CharField(max_length=45, blank=True, null=True)
    owner = models.CharField(max_length=45, blank=True, null=True)

class GeoLocation(models.Model):
    id = models.CharField(primary_key=True, max_length=32,
                          default=generate_uuid)
    display = models.ForeignKey(
        Displays, on_delete=models.CASCADE, blank=True, null=True)
    lat = models.DecimalField(max_digits = 30, decimal_places=20, blank=True, null=True)
    lon = models.DecimalField(max_digits = 30, decimal_places=20, blank=True, null=True)

I think the issue is here, how to get efficiently geolocation?

class DisplaySerializer(serializers.ModelSerializer):
    
    geolocation = serializers.SerializerMethodField()
    
    def get_geolocation(self, obj):
        gl = GeoLocation.objects.filter(display = obj)
        gll = list(gl.values)
        return gll

    class Meta:
        model = Displays
        fields = "__all__"

>Solution :

Use a nested serializer so that you don’t have to return the nested data via a method

class GeoLocationSerializer(serializers.ModelSerializer):

    class Meta:
        model = GeoLocation
        fields = "__all__"


class DisplaySerializer(serializers.ModelSerializer):
    
    geolocation_set = GeoLocationSerializer(many=True)

    class Meta:
        model = Displays
        fields = ["name", "owner", "geolocation_set"]

Then in your view, use prefetch_related to get the nested data in a single query. This will reduce your queries to only two

@permission_classes([AllowAny])
class DisplaysList(generics.ListAPIView):
    queryset = Displays.objects.all().prefetch_related("geolocation_set")
    serializer_class = serializers.DisplaySerializer