title: Rails find where ALL associated records meet condition
body: <p>I'm trying to get all the clients that have doctors associated BUT none of them has started their first session (one client has_many doctors and can have first sessions with each of them).</p>
  
  <p>So far I have:</p>
  
  <pre><code>@clients =  Client.joins(:doctors).where('doctors.first_session IS NULL').order('clients.id DESC')
  </code></pre>
  
  <p>But this doesn't work when a client has for example 2 doctors. the first doctor.first_session = null but the second one is not. This case will return the client and it don't want it to.</p>
  
  <p>Any ideas?</p>