Incorporate Cart's relations into ActiveRecord logic

[shippy]

This is a thought, not a critical issue.

Currently, Cart exists outside of the logical ActiveRecord relations (has_many :cart_reservations, belongs_to :user) because it is never stored in a database. Consequently, the application logic is confusing to work with and maintenance is hard. (It is also possible that the application is to vulnerabilities. For example, no methods that use @cart.items, which is an array of CartReservation.id fields, ever check if @cart.reserver == CartReservation.reserver before applying changes to all @cart.items.)

This RailsCast episode and this StackOverflow topic suggest a way to take advantage of ActiveRecord without the database connection.

Alternatively, the Cart logic could be preserved but be included in User model, with appropriate has_many :cart_reservations designation. This would work, as we anticipate 1-to-1 relation between User and Cart anyway.

[squidgetx]

irrelevant via #587