title: Rails active record querying association with 'exists'
body: <p>I am working on an app that allows Members to take a survey (Member has a one to many relationship with Response). Response holds the member_id, question_id, and their answer. </p>
  
  <p>The survey is submitted all or nothing, so if there are any records in the Response table for that Member they have completed the survey.</p>
  
  <p>My question is, how do I re-write the query below so that it actually  works? In SQL this would be a prime candidate for the EXISTS keyword.</p>
  
  <pre><code> def surveys_completed
      members.where(responses: !nil ).count
   end 
  </code></pre>