              elif model_field.rel:
                  field = self.get_related_field(model_field)
              else:
                  field = self.get_field(model_field)
              field.initialize(parent=self, model_field=model_field)
              ret[model_field.name] = field
          return ret
  
      def get_nested_field(self, model_field):
          """
          Creates a default instance of a nested relational field.
 
</s> remove             if model_field.rel and nested:
 </s> add             if is_pk:
                field = self.get_pk_field(model_field)
                is_pk = False
            elif model_field.rel and nested: </s> remove         Creates a default instance of a basic field.
 </s> add         Creates a default instance of a basic non-relational field.
            if field is not None:
                field.initialize(parent=self, model_field=model_field)
                ret[model_field.name] = field

              elif model_field.rel:
                  field = self.get_related_field(model_field)
              else:
                  field = self.get_field(model_field)
            if field is not None:
                field.initialize(parent=self, model_field=model_field)
                ret[model_field.name] = field

            if field is not None:
                field.initialize(parent=self, model_field=model_field)
                ret[model_field.name] = field

          return ret
  
      def get_nested_field(self, model_field):
          """
          Creates a default instance of a nested relational field.
keep keep keep keep replace replace keep keep keep keep keep
 Fix bug where pk could be set in post data 
rest_framework/serializers.py