I really don't see the point of this question. AR is an API for querying RDBMS and that's how you do it in AR.
If you want to do redis you'll have to either write it yourself at the driver level or find a similar abstraction to AR for Redis... I think DataMapper had a redis adapter. If there is a universal way to do this for any data store it is likely in DataMapper, but the basic pattern to follow when creating your own would be to look at how AR implements find_each/find_in_batches and do it for your store of choice.