Thank you for the fast reply. I wanted to use hash at first so the key is group and the value is an array of values. But the problem is that this format should be of JSON that will be sent to Mongo and Mongo does not allow dots and the dollar sign to be in the key name. But we want to allow those special chars to be in groups so we had to rechange the structure and they can't be keys. I can't see any other structure than the one I suggested in the post.

Anyway, I know that I keep pushing it each time, my question here is how I can make sure it gets to the right place? I'll have to iterate over the array and that does not feel efficient. Maybe you can suggest how the structure should look like? Thanks again!