title: Doesn't Show the added field using SerializerMethodField
body: <p>These are my models.py</p>
  
  <pre><code>class Grade(models.Model):
      grade = models.CharField(max_length=255, primary_key=True)
  class Student(models.Model):
      name = models.CharField(max_length=255) 
      grade = models.ForeignKey(grade, on_delete=models.CASCADE)
      rollno = models.BigIntegerField()
  </code></pre>
  
  <p>What I am trying to do is make a serializer which gives me list(to implement get and delete method) of all students to a particular grade using the url: 'check/{str:pk}/'.
  In my serializer I am trying to combine the two models and display them together like below:</p>
  
  <pre><code>class MySerializer(serializers.ModelSerializer):
      allStudents = serializers.SerializerMethodField()
      class Meta:
          model = Grade
          fields = ("grade", "allStudents")
  
      def get_allStudents(self, obj):
          s_list = []
          fields = ['name', 'grade', 'rollno']
          for stds in obj:
              s_list.append(model_to_dict(stds, fields=fields))
          return s_list
  </code></pre>
  
  <p>And my views.py for the same looks like following:</p>
  
  <pre><code>class IndividualGrade(generics.RetrieveUpdateDestroyAPIView):
      '''
      PUT check/{grade}/
      GET check/{grade}/
      DELETE check/{grade}/
      '''
      queryset = Grade.objects.all()
      serializer_class = MySerializer
  
      def get(self, request, *args, **kwargs):  
          try:
              s1 = Grade.objects.get(grade=kwargs["pk"])
              all_stds = Student.objects.filter(grade=s1) 
              a_grade = Grade.objects.get(grade=s1)
              return Response(MySerializer(a_grade, all_stds ).data)
          except Grade.DoesNotExist:
              return Response(
                  data={
                      "message": "Grade with id: {} does not exist".format(kwargs["pk"])
                  },
                  status=status.HTTP_404_NOT_FOUND
              )
  
      def delete(self, request, *args, **kwargs):        
          try:
              Grade.objects.get(grade=kwargs["pk"]).delete()
              return Response(status=status.HTTP_204_NO_CONTENT)            
          except Grade.DoesNotExist:
              return Response(
                  data={
                      "message": "Grade with id: {} does not exist".format(kwargs["pk"])
                  },
                  status=status.HTTP_404_NOT_FOUND
              )
  </code></pre>
  
  <p>This gives me the following error:</p>
  
  <pre><code>"message": "Grade with id: grade9 does not exist"
  </code></pre>
  
  <p>While the entry is definitely there. Because I can delete it, additionally it doesn't show me the added field.</p>
  
  <p>I am really naive when it comes to serializers which aren't straight away.</p>
  
  <p>Still not sure what am I doing wrong.
  Also, I was wondering if I could use another serializer in SerializerMethodFiled</p>
  
  <p><strong>EDIT</strong>
  1. To update
  PUT REQUEST:<code>http://127.0.0.1:8000/check/grade5/</code></p>
  
  <p>JSON input: </p>
  
  <pre><code>{
      "grade": "grade5",
      "allstudents": [
          {
              "name": "Shub",
              "grade": "grade5",
              "rollno": 4
          }
      ]
  }
  </code></pre>