      def default_fields(self, nested=False):
          """
          Return all the fields that should be serialized for the model.
          """
          # TODO: Modify this so that it's called on init, and drop
          #       serialize/obj/data arguments.
          #
          #       We *could* provide a hook for dynamic fields, but
          #       it'd be nice if the default was to generate fields statically
          #       at the point of __init__
  
          cls = self.opts.model
          opts = get_concrete_model(cls)._meta
          pk_field = opts.pk
          while pk_field.rel:
 
</s> remove     def default_fields(self, nested=False):
 </s> add     def get_default_fields(self): </s> remove         fields = self.get_fields(nested=bool(self.opts.depth))
 </s> add         fields = self.get_fields() </s> remove     def default_fields(self, nested=False):
 </s> add     def get_default_fields(self): </s> remove         fields = self.default_fields(nested)
        for key, val in fields.items():
 </s> add         for key, val in self.default_fields.items(): </s> remove         plus the set of fields returned by default_fields().
 </s> add         plus the set of fields returned by get_default_fields().
      def default_fields(self, nested=False):
          """
          Return all the fields that should be serialized for the model.
          """






  
          cls = self.opts.model
          opts = get_concrete_model(cls)._meta
          pk_field = opts.pk
          while pk_field.rel:
keep keep keep keep replace replace replace replace replace replace keep keep keep keep keep
 Cache default fields per serializer instance for improved performance 
rest_framework/serializers.py