Mahout - Item Similarity, but exclude Items a User has already "bought"

Question

I want to create a video recommender, which recommends via similarity. The challenge is, that I want to exclude videos that the user has already seen. This seems like a pretty obvious case to me, but I don't find it covered.

Any hint is appreciated!

Answer 1

This is the default behavior of any recommender, to not return items that already appears in the user's input vector. Certainly it's how the ones I have worked on work.

Do you really mean how? It's just a filtering step. You just don't consider any item that exists when you look it up in the input.

You can always post-process results any way you want beyond this. Mahout/Myrrix both have an IDRescorer abstraction that lets you inject whatever logic you want to remove or boost items in the results. Here's a writeup on rescoring that applies to both.