How to use count() and sum() in serializer?

Question

I want to calculate the average rating on sum_of_rating/number_of_rating.

class SearchWorkingProfessionals(APIView):
    def post(self,request,format=None):
        tags = request.data.get('tag_arr')
        city_name = request.data.get('city_name')
        tags_list = tags.split(',')
        ws = WorkSamplesModel.objects.filter(business_account__serviceareasmodel__city_name=city_name,
        business_account__professiontagsmodel__tag_name__in=tags_list,
        is_business_card_image=True).distinct()
        serializer = SearchWorkingProgessionalsSerializer(ws,many=True)
        resp = {'resp':serializer.data}
        return Response(resp)

class FeedbackModel(models.Model):
    feedback_text = models.CharField(max_length=20)
    rating = models.IntegerField()
    business_account = models.ForeignKey(BusinessAccountModel,on_delete=models.CASCADE)
    
    class Meta:
        db_table = 'feedback'

This is my current serializer

class SearchWorkingProgessionalsSerializer(serializers.Serializer):
    business_account_id = serializers.IntegerField()
    first_name = serializers.CharField(source='business_account.user.first_name')
    last_name = serializers.CharField(source='business_account.user.last_name')
    profile_pic = serializers.ImageField(source='business_account.user.profile_pic')
    business_title = serializers.CharField(source='business_account.business_title')
    business_description = serializers.CharField(source='business_account.business_description')
    status = serializers.CharField(source='business_account.status')
    note = serializers.CharField(source='business_account.note')
    work_sample_image = serializers.ImageField()
    work_sample_description = serializers.CharField(max_length=1000)

class WorkSamplesModel(models.Model):
    work_sample_image = models.ImageField(blank=True,null=True,upload_to="work_samples")
    work_sample_description = models.CharField(max_length=1000)
    is_business_card_image = models.BooleanField(default=False)
    business_account = models.ForeignKey(BusinessAccountModel,on_delete=models.CASCADE)

How to calculate the average rating using this serializer?

Answer 1

The only option you have is this case is to use a SerializerMethodField. Something like this should work (if I understood what you asked for):

from django.db.models import Avg

class SearchWorkingProgessionalsSerializer(serializers.Serializer):
    count_feedbacks = serializer.SerializerMethodField()
    average_rating = serializer.SerializerMethodField()

    # Returns the number of ratings
    def get_count_feedbacks(self, instance):
        return instance.business_account.feedbackmodel_set.count()  

    # Returns the average rating
    def get_average_rating(self, instance)
        return instance.business_account.feedbackmodel_set.aggregate(average_rating=('rating'))['average_rating']

Answer 2

You can always write a serializers.SerializerMethodField() to return dynamically calculated fields in serializers.

In your specific case, you should post WorkSamplesModel as well for more clarity.

Answer 3

Following code work for me..!

from django.db.models import Avg
...


class SearchWorkingProfessionalsSerializer(serializers.Serializer):
    business_account_id = serializers.IntegerField()
    first_name = serializers.CharField(source='business_account.user.first_name')
    last_name = serializers.CharField(source='business_account.user.last_name')
    profile_pic = serializers.ImageField(source='business_account.user.profile_pic')
    business_title = serializers.CharField(source='business_account.business_title')
    business_description = serializers.CharField(source='business_account.business_description')
    status = serializers.CharField(source='business_account.status')
    note = serializers.CharField(source='business_account.note')
    work_sample_image = serializers.ImageField()
    work_sample_description = serializers.CharField(max_length=1000)
    feedback_count = serializers.SerializerMethodField('get_feedback_count')
    average_rating = serializers.SerializerMethodField()

    def get_feedback_count(self, instance):
        return instance.business_account.feedbackmodel_set.count()

    def get_average_rating(self, instance):
        return instance.business_account.feedbackmodel_set.aggregate(average_rating=Avg('rating'))['average_rating']