title: How to exclude a collection of records with ActiveRecord?
body: <p>I have the following:</p>
  
  <pre><code>= select :client, :id, User.all.map {|u| [u.username, u.id]}, include_blank: "Add a client by username"
  </code></pre>
  
  <p>I'd like to exclude all records from User.all that match <code>current_user.manager_users</code>. The point is so that the select box doesn't show users that are already added to the manager_users array which is a <code>has_many</code> collection.</p>