If the sub_id is only intended for display purposes, then we should keep it in the view. And, the population already has a mechanism for knowing which index it is. With this code, there are 3 ways at the model level to know; the sub_id, the index in the collection, and the population_index. The third can easily be replaced by the second. If you had an `index` method on population which called `@collection.indexOf(this)` I'd like that better than what we have here.