          return PrimaryKeyRelatedField()
  
      def get_field(self, model_field):
          """
          Creates a default instance of a basic field.
          """
          return Field()
  
      def restore_object(self, attrs, instance=None):
          """
 
</s> remove             field.initialize(parent=self, model_field=model_field)
            ret[model_field.name] = field
 </s> add             if field is not None:
                field.initialize(parent=self, model_field=model_field)
                ret[model_field.name] = 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:
        Creates a default instance of a basic non-relational field.
          return PrimaryKeyRelatedField()
  
      def get_field(self, model_field):
          """
        Creates a default instance of a basic non-relational field.
          """
          return Field()
  
      def restore_object(self, attrs, instance=None):
          """
keep keep keep keep replace keep keep keep keep keep
 Fix bug where pk could be set in post data 
rest_framework/serializers.py