Django Rest Framework - automatically annotate queryset

Question

I use one Serializer in many different places in my project. I need to use one annotation but the problem is that I don't want to annotate it in all views so I would like to do universal annotation in the Serializer itself.

It is possible?

Now I need to do this before every serialization:

City.objects....filter....annotate(
                number_of_users_here_now=Count('current_userprofiles'))

I tried:

class NumberOfUsersInCityNowField(serializers.Field):
    def to_native(self, value):
        count = value.annotate(
            number_of_users_here_now=Count('current_userprofiles'))['current_userprofiles__count']
        return count


class CityMapSerializer(serializers.ModelSerializer):
    number_of_users_here_now = NumberOfUsersInCityNowField()

    class Meta:
        model = City
        fields = ('place_id', 'lat', 'lng', 'number_of_users_here_now', 'formatted_address')

This Serializer returns:

AttributeError at /api/ajax-check-trip-creation Got AttributeError when attempting to get a value for field number_of_users_here_now on serializer CityMapSerializer. The serializer field might be named incorrectly and not match any attribute or key on the City instance. Original exception text was: 'City' object has no attribute 'number_of_users_here_now'.

EDIT

class NumberOfUsersInCityNowField(serializers.PrimaryKeyRelatedField):
    def get_queryset(self):
        return City.objects.annotate(
        number_of_users_here_now=Count('current_userprofiles'))

class CityMapSerializer(serializers.ModelSerializer):
    # number_of_users_here_now = serializers.IntegerField()
    number_of_users_here_now = NumberOfUsersInCityNowField()
    class Meta:
        model = City
        fields = ('place_id', 'lat', 'lng', 'number_of_users_here_now', 'formatted_address')

But

serializers.CityMapSerializer(City.objects.all()[:3],many=True).data

still returns:

AttributeError: 'City' object has no attribute 'number_of_users_here_now'

No, that's not a native field. I need to add count of userprofiles for each city. — Milano
– Milano, Commented Jan 8, 2018 at 15:47

neverwalkaloner · Accepted Answer · 2018-01-08 16:07:53Z

1

You can just use queryset's count method with SerializerMethodField:

class CityMapSerializer(serializers.ModelSerializer):
    number_of_users_here_now = SerializerMethodField()

    def get_number_of_users_here_now (self, obj):
        return obj.current_userprofiles.count()

UPD

Also to avoid n+1 queries you can try to implement get_queryset method of NumberOfUsersInCityNowField serializer:

class NumberOfUsersInCityNowField(serializers.PrimaryKeyRelatedField):
    def get_queryset(self):
        return City.objects.annotate(
        number_of_users_here_now=Count('current_userprofiles'))['current_userprofiles__count']

edited Jan 8, 2018 at 16:07

answered Jan 8, 2018 at 15:55

neverwalkaloner

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5 Comments

Milano Over a year ago

I wan't to avoid doing this since it access the database for every city, not just once as it is with annotating.

neverwalkaloner Over a year ago

@MilanoSlesarik yes you are right, try to inherited from PrimaryKeyRelatedField and override get_queryset method.

Milano Over a year ago

Your solution seems to be what I want but unfortunately it returns the same error. I've added the code at the bottom of the question.

Milano Over a year ago

The City is a foreign key in model UserProfile with related_name='current_userprofiles'

neverwalkaloner Over a year ago

@MilanoSlesarik sorry, cannot suggest you anything better. Iif you will not find the solution probably you should try mixin class which implements annotation and add it to all your view.

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