title: Find records which assoicated records do not belong to certain record
body: <p>In my system I have a following structure:</p>
  
  <pre><code>class Worker
    has_many :worker_memberships
  end
  
  class WorkerMembership
    belongs_to :worker
    belongs_to :event
  end
  
  class Event
    has_many :worker_memberships
  end
  </code></pre>
  
  <p>Imagine I have a certain <code>@event</code>. How can I find all <code>workers</code> that have NO <code>worker_memberships</code> belonging to this <code>@event</code>?</p>