How do I validate that a model object’s attribute is a valid foreign
key? The problem is, I can’t check if the attribute is a valid foreign
key if the attribute doesn’t even exist.
For example, every employee must be in a department. In the following
code, if an employee’s department_id is not present then
Department.find_by_id(department_id) might cause problems, yes?
class Employee < ActiveRecord::Base
department_id should be present, a positive integer, and a valid
foreign key to the departments table. This is a common thing to want
to validate. How do you do it?