title: has_one without belongs_to in ecto
body: <p>Imagine I have two schemas: <code>User</code> and <code>Job</code>, with <code>Job</code> being one of <code>Firefighter</code>, <code>Teacher</code>, <code>Police Officer</code>, etc.</p>
  
  <p>Each <code>User</code> can only have one <code>Job</code> (<code>has_one</code>), but each <code>Job</code> will have an infinite number of <code>User</code>, and I'm really not even concerned with being able to find the <code>User</code> from a given <code>Job</code>.</p>
  
  <p>I can't figure out has to define a <code>has_one</code> on the <code>User</code> without <em>also</em> including a <code>belongs_to</code> on the <code>Job</code> and still be able to preload the association. I know I could define this as a <code>many_to_many</code> using a <code>UserClass</code> table, but that seems like overkill for something this simple. I'm struggling to wrap my head around my options here.</p>