During hard times where JOINS are not allowed and performance matters (i.e. don't create objects unless is really necessary), too many times I've been writing ActiveRecord find all, loop on the collection and return an hash where key is some column of the object...

Classic Scenario:

find :messages, :include :sender, :include :recipient

This will usually result in a reasonable query, but as the complexity grows things can get out of control with AR generating unnecessary db calls.

Optimized Scenario:

find :messages

find :users with ids in messages.map(&:sender_id) + messages.map(&:recipient_id)

and finally loop on messages and assign sender and recipient names as attr of the message, but this is still expensive.

Optimized Scenario V2:

find :messages