FIRST

Here's an example refactoring from http://www.workingwithrails.com/forums/4-ask-a-rails-expert/topics/789-optimising-code

models.rb

class Role < ActiveRecord::Base
  ADMIN = 'admin'

  belongs_to :user
  belongs_to :building

  named_scope :by, proc { |role|
    { :conditions => ['name = ?', role] }
  }
end

class User < ActiveRecord::Base
  has_many :roles
  has_many :buildings, :through => :roles
end

new.rb

# The new way...
class Building < ActiveRecord::Base
  has_many :roles
  has_many :users, :through => :roles do
    # Association extension
    def with_role(name)
      scoped(:conditions => ['roles.name = ? AND roles.id = ?', name, proxy_owner.id])
    end
  end

  def find_admins_of
    users.with_role(Role::ADMIN)
  end
end

old.rb

# The old way...
class Building < ActiveRecord::Base
  def find_admins_of
    roles = self.roles
    admins = []
    for role in roles
      admins << User.find(role.user_id) if role.role_type_id == Role::ADMIN
    end
    admins
  end
end

test.rb

require 'test/unit'

class AdminTest < Test::Unit::TestCase
  def test_with_role_filters_by_role
    # Create test objects
    admin     = User.create!
    not_admin = User.create!
    building  = Building.create!

    # Give the admin user the right role
    admin.roles.create! :name => Role::ADMIN, :building => building

    # Assert that the filter works
    assert_equal [admin], building.users.with_role(Role::ADMIN)

    # ...or if you prefer:
    assert_equal [admin], building.find_admins_of
  end
end