core: Improve validations for quantity
Fix to preload associations when created order record
core: Change method to preload for order

includes => preload
core: Refactor the cart methods
core: Change timing of validations
Refactor method names
core: Add a record transaction
rubocop: Refactor a method
Change greatly the flow of validations
rubocop: Refactor to improve the ABC metrics
Rails 3.x: Fix a problem that cart items are not cached
Fix to work the test case
Merge pull request #47 from hyoshida/improve-validations-for-quantity

Improve validations for quantity