title: Rails, How to determine if a Nested Record Set Exists
body: <p>I have the following two models:</p>
  
  <pre><code>Project, has_many ProjectParticipants
  ProjectParticipants, belongs_to Project
  </code></pre>
  
  <p>How can I request to determine given these 5 ProjectParticipants, do they belong to a Project?</p>
  
  <p>Also, it should be strictly those 5, not more or less.</p>
  
  <p>Any thoughts on how to elegantly solve for this type of count?</p>