scopes_rock.rb

class Item < ActiveRecord::Base
  attr_accessible :description, :owner_id, :title, :image_url, :weekly, :daily,
                  :amazon_url, :image, :brand, :component
  belongs_to :owner, class_name: 'User'

  COMPONENTS = [:lens, :body, :accessory].each do |component|
    scope component.to_sym, where(component: component)
  end
  BRANDS     = [:canon, :nikon, :sony, :olympus, :other].each do |brand|
    scope brand.to_sym, where(brand: brand)
  end
end

1.9.3p327 :005 > Item.canon.body.collect(&:title)
  Item Load (0.3ms)  SELECT "items".* FROM "items" WHERE "items"."brand" = 'canon' AND "items".
"component" = 'body'
 => ["Canon 60D Body", "Canon Rebel XTi 10.1 MP Body"]