How to use _set in SerializerMethod for instance? - django

Question

I want to calculate the average rating by using SerializerMethodField().
The error in the following code is AttributeError: 'FeedbackModel' object has no attribute 'aggregate'
I think _set is missing but I don't know where to put it..!
class FeedbackSerializer(serializers.ModelSerializer):
feedback_by_user_profile_pic = serializers.ImageField(source='feedback_by.profile_pic')
average_rating = serializers.SerializerMethodField()
def get_average_rating(self,instance):
return instance.aggregate(average_rating=Avg('rating'))['average_rating']
class Meta:
model = FeedbackModel
fields = ['feedback_text','rating','date','feedback_by_user_profile_pic','average_rating']
Feedback Model
class FeedbackModel(models.Model):
feedback_text = models.CharField(max_length=1000)
rating = models.IntegerField()
date = models.DateField(auto_now=True)
feedback_by = models.ForeignKey(UserModel,on_delete=models.CASCADE)
business_account = models.ForeignKey(BusinessAccountModel,on_delete=models.CASCADE)
class Meta:
db_table = 'feedback'
BusinessAccountModel
class BusinessAccountModel(models.Model):
business_title = models.CharField(max_length=70)
business_description = models.CharField(max_length=500)
status = models.CharField(max_length=100)
note = models.CharField(max_length=200)
user = models.OneToOneField(UserModel,on_delete=models.CASCADE)
class Meta:
db_table = 'wp_business_acc'
BusiAccSerializer
class BusiAccSerializer(serializers.ModelSerializer):
class Meta:
model = BusinessAccountModel
fields = '__all__'

Answer 1

I think you need to add the average_rating field in the BusiAccSerializer, not in the FeedbackSerializer.
First you have to set the related_name attribute in the FeedbackModel.
class FeedbackModel(models.Model):
...
# here I added the `related_name` attribute
business_account = models.ForeignKey(BusinessAccountModel,on_delete=models.CASCADE, related_name="feedbacks")
And then in the BusiAccSerializer,
class BusiAccSerializer(serializers.ModelSerializer):
average_rating = serializers.SerializerMethodField(read_only = True)
def get_average_rating(self, obj):
return obj.feedbacks.aggregate(average_rating = Avg('rating'))['average_rating']
class Meta:
model = BusinessAccountModel
fields = (
'business_title', 'business_description', 'status', 'note', 'user', 'average_rating',
)

Answer 2

First of all, you've indicated that you need an average rating for a business account, but you can not get an average rating for an account without having a concrete business account, so you need to do it in the business account serializer.
David Lu has already answered how to do it in the BusiAccSerializer, but I have something to add:
What you've trying to do is to use a serializer method field to add some aggregated data to the output. This way of solving your problem has a major drawback: when you will try to serialize a list of BusinessAccountModels, the serializer will do a separate database call for each business account and it could be slow. You better need to specify an annotated queryset in your view like this:
BusinessAccountModel.objects.all().annotate(average_rating=Avg('feedbacks__rating'))
Then you will be able to use the result of calculation as a regular field in your serializer:
class BusiAccSerializer(serializers.ModelSerializer):
...
average_rating = serializers.FloatField(read_only=True)
This way there will be no additional database queries done by the serializer.