Say you have several ‘nested’ has_many relations:

class State < ActiveRecord::Base
  has_many :cities
end

class City < ActiveRecord::Base
  has_many :streets
  belongs_to :state
end

class Street < ActiveRecord::Base
  has_many :houses
  belongs_to :city
end

class House < ActiveRecord::Base
  belongs_to :street
end

How do you get all of the Houses in virginia?

Well, you could do this:

virginia.cities.collect{|c| c.streets}.flatten.uniq.collect{|s| s.houses}.flatten.uniq

... but that's epically lame. It looks like shit and produces an metric ton of queries and as a result is highly inefficient.

How about this:

House.find(
  :all,
  :include => {:street => {:city => :state}},
  :conditions => {'states.id' => virginia.id}
)

This is a single query with joins where appropriate, and it's a finder on House which is what you're getting anyways. Makes sense, no?

script_console_running = ENV.include?('RAILS_ENV') &&
                         IRB.conf[:LOAD_MODULES] &&
                         IRB.conf[:LOAD_MODULES].include?('console_with_helpers')
rails_running = ENV.include?('RAILS_ENV') &&
                !(IRB.conf[:LOAD_MODULES] &&
                IRB.conf[:LOAD_MODULES].include?('console_with_helpers'))
irb_standalone_running = !script_console_running && !rails_running
if script_console_running
  require 'logger'
  Object.const_set(:RAILS_DEFAULT_LOGGER, Logger.new(STDOUT))
end

You see, if you drop this little gem in ~/.irbrc you’ll start getting the SQL output from your various ruby commands. This is an excellent debugging tool, but it’s an even better learning tool.

To the novice the difference between the following two statements may not be readily apparent…

User.find(:first).posts.each{|p| p.comments.do_something}
User.find(:first, :include => {:posts => :comments}).posts.each{|p| p.comments.do_something}

…but as soon as they sit and watch what scrolls by in the terminal they will *very* quickly realize that one of them is *very* wrong.

I submit that the next time you are teaching anyone the basics of AR that this be the very first thing that you introduce them to.