I guess your using the globalize gem? In this case: It seems that the serialize method of globalize is setting the class_name_or_coder attribute to Object which let's active record then print the deprecation warning.

This is an already known problem, see Github Issue. As I understand globalize will provide a fix for this with this: serialization

Since I have the same problem I would suggest to wait for a new globalize release which fixes the generation of this deprecation warning.