Model names distinct from database fields

Naming a method in an ActiveRecord object after the DB column causes
issues with Rails validations such as presence and uniqueness. Even if
the object does not have those validations now, wrapping an AR attribute
limits people in the future arbitrarily.

Instead of wrapping a column, instead name the column more simply. For
example, if you need to expose the receipt string as an `AppleReceipt`,
store the value as `encoded_receipt` or `receipt_string`, then provide a
`receipt` method that handles the wrapping.