title: Is a migration necessary for generating an ID in a has_many/belongs_to relationship?
body: <p>I have a rails app with two related classes. Pet <code>has_many</code> Toys, and a Toy <code>belongs_to</code> a Pet. </p>
  
  <p>In order to have an index of Toys by Pets (meaning for every toy I want to have an index identifier stating which Pet it belongs to), do I need to add a migration adding the Pet_ID column to each Toy (to the Toys table), or is this already automatically done by Rails by the <code>has_many</code>-<code>belongs_to</code> relationship?</p>