title: How to build EXISTS clause in sequelize
body: <p>I have a model where there are 3 entities:</p>
  
  <pre><code>User
  Project
  Contributor
  </code></pre>
  
  <p>A Project has a Owner (who is a User) and has multiple Contributors (who are Users). In my dashboard, I want to list a User's projects i.e. all Projects where the User is a Owner or a Contributor. </p>
  
  <p>Finding owned projects using <code>Project.findAll({where: [owner: user]})</code> is trivial. How do I modify this to find out the Projects where the user is also a contributor? This would (in the database query) translate to a sub-query using EXISTS clause.</p>