in src/main/scala/com/spotify/bdrc/pipeline/MinItemPerUser.scala [30:36]
def scalding(input: TypedPipe[Rating]): TypedPipe[Rating] = {
input
.groupBy(_.user)
// Reduce items per key by picking the side with lower score for each pair of input
.reduce((x, y) => if (x.score < y.score) x else y)
.values
}