Count by group optimization.

Hello RoR people,

First time here.

I have a question about calculations in rails.

For example i have two tables:

categories
  id
  name

and questions:
  id
  category_id
  name

How I can count questions in each category?

Please don't give me recepies like: :include => [:questions] or
something like: @categories.each { |c| c.questions.count } Because
those methods is not so much optimized, to run them on a production
server.

Thank you,
Dmitry for Estonia.