Create recommendation list across ratables

[Nuru]

Let's say, for example, we are allowing users to rate songs and artists. I would like to be able to get a single, ordered list of recommendations for a user that includes both songs and artists based on the likelihood that the user will like it, regardless of its class.

I would be willing to build the list myself by calling recommended_songs and recommended_artists and merging the lists if I could get the associated scores so I would know how to relate the top songs to the top artists.