title: Rails exclusion query with active record -- Rails 3.1
body: <p>How would I use active record in this case:</p>
  
  <p>I have two tables: Users, Admins</p>
  
  <p>Admins can be users also and thus have a user_id in their table.</p>
  
  <p>I'd like to <strong>Select all users that DO NOT have a user_id set in the admins table</strong></p>
  
  <p>NOTE: This is just an example case. If this was a real case then of course I wouldn't have two different tables for users and admins.</p>
  
  <p>Edit: Again, I know the semantics of doing it like this is terrible db design -- but it's just an dummy case.</p>