Allow for multiple rater types (Multiple Models as the source of a rating)

[bsbodden]

Just like you have a rateable_id and a rateable_type we could have a rater_type to go along rater_id so that multiple models could act as a rater.

The example I have is a course management and delivery app where there are Authors that can rate elements of the material for a course and Students that can rate a delivery of a course.