Just a quite note to myself because I know I'm going to forget this a lot. The following is not optional in Rails 5 (unless you specify that it should be):
Class ModelName < ApplicationRecord belongs_to :other_model end
Update: This was probably a little unclear. What I mean is, if you create an association that says that a model belongs to another, if you try to create a record without saying what it belongs to, it will fail.