add layouts of cart
javascripts of depot
stylesheets of line_item
method changed in controller
add method in db migrate
changed the view of carts
add relations of carts and line_items
unit test of cart and line_items
add exception in for migration
helper method
add fixtures in carts and line_items
add functions for test in carts and line_itepp/helpers/carts_helper.rb
add view of line_items
unit test of helpers