get count from related table data in django rest framework

Question

I have two tables

class student(models.Model):
    frist_name = models.CharField(max_length=255)
    last_name = models.CharField(max_length=255)


class subject(models.Model):
    student = models.ForeignKey(student)
    sub_name = models.CharField()

I want student list and subject count in serializer

my serializer

classs SubjectSerializers(serializers.ModelSerializer):
    class Meta:
        model = JobPosting
        fields = ('id','sub_name')

class StudentSerializers(serializers.ModelSerializer):
    sub = SubjectSerializers(source = 'student')
    class Meta:
        model = JobPosting
        fields = ('id','first_name', 'last_name','sub')

How can i get subject count for every student in serializer, Now i am geting subject table data but i want count like this

"detail": [{
        "id": 680,
        "first_name": "riya",
        "last_name": "tri",
        "subject_count": 5
      }],

andrroy · Accepted Answer · 2017-05-05 18:36:14Z

3

You can achieve this by using a serializer method field

Then StudentSerializer becomes the following:

class StudentSerializers(serializers.ModelSerializer):
    sub = SubjectSerializers(source = 'student')
    subject_count = serializers.SerializerMethodField()

    class Meta:
        model = JobPosting
        fields = ('id','first_name', 'last_name','sub')

    def get_subject_count(self, student):
        return Subject.objects.filter(student=student).count()

answered May 5, 2017 at 18:36

andrroy

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1 Comment

coler-j Over a year ago

What would the performance implications of this be? Wouldn't this perform a separate query for each JobPosting object?

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