title: Cannot pass the value from kwargs in POST method django rest framework
body: <p>When I try to pass the value of course_id using perform_create method, It shows this error...</p>
  
  <pre><code>ValueError at /course/9f77f4a9-0486-44f3-8bea-4908adb7d3ca/add/
  Cannot assign "'9f77f4a9-0486-44f3-8bea-4908adb7d3ca'": "Content.course_id" must be a "Course" instance.
      Request Method: POST
      Request URL:    http://127.0.0.1:8000/course/9f77f4a9-0486-44f3-8bea-4908adb7d3ca/add/
      Django Version: 2.2.5
      Exception Type: ValueError
      Exception Value:    
      Cannot assign "'9f77f4a9-0486-44f3-8bea-4908adb7d3ca'": "Content.course_id" must be a "Course" instance.
  </code></pre>
  
  <p>This is my views.py</p>
  
  <pre><code>class ContentAdd(generics.ListAPIView, mixins.CreateModelMixin):
      queryset = Content.objects.all()
      serializer_class = ContentSerializer
  
      def post(self, request, *args, **kwargs):
          saveData = self.create(request, *args, **kwargs)
          response_data = {}
          response_data["data"] = saveData.data
          response_data["errors"] = {}
          return Response(response_data, status=status.HTTP_201_CREATED)
  
      def perform_create(self, serializer):
          id = self.kwargs.get("course_id")
          serializer.save(course_id=id)
  </code></pre>
  
  <p>This is my serializers.py</p>
  
  <pre><code>class ContentSerializer(serializers.ModelSerializer):
      class Meta:
          model = Content
          fields = [
              'title',
              'description',
              'serial',
              'file',
              'file_type',
          ]
  </code></pre>
  
  <p>This is my model.py</p>
  
  <pre><code>class Content(models.Model):
      content_id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
      course_id = models.ForeignKey(Course, on_delete=models.CASCADE, related_name="content_course_id", to_field='course_id')
      file = models.FileField(upload_to=gen_pic_path,blank=False, null=False)
      file_type = models.BooleanField(blank=False, null=False)    # content/attachment
      serial = models.IntegerField()
      title = models.CharField(max_length=200)
      description = models.TextField()
  </code></pre>
  
  <p>This is url path</p>
  
  <pre><code>path('course/&lt;course_id&gt;/add/', ContentAdd.as_view()),
  </code></pre>