Nice job on figuring that out! Good use of `inject`.

Couple quick comments, just for your reference:

1. You can shorten the `map`s you're doing like this: `map(&:rating)`. Not required, but a handy little trick to know about.
2. You don't need the `each` before the `inject`, `ratings` is already an array, so you can `inject` directly. Also, arrays have a `sum` method, so you can also do `ratings.sum`.
3. Might consider moving this into a `karma` method on `User`.

Great job tackling this!