update_attribute from STI model does not work

Just for the record, this is nothing to do with STI.

This happens if - You load a model - You change the association (build, create, or =) - You then change the foreign key

As you may have already noticed, if foo belongs_to bar changing :bar_id will not cause the value of foo.bar to change if the association is already loaded and this is slightly connected. During the save activerecord thinks that the current value of the association is foo.bar and so overwrites the change you made to the foreign key