How to join a table and count records in Rails 3?

ruby ruby-on-rails-3 arel

To answer my own question:

Collection.joins(:coins).group("coins.collection_id").having("count(coins.id) > 2")

Hat tip to KJF who asked this similar question and to krakover for answering it.

ruby ruby-on-rails-3 arel

Add counter_cache columns and query them.

http://railscasts.com/episodes/23-counter-cache-column