title: Joins with Sequelize
body: <p>I have these associations:</p>
  
  <pre><code>Room hasMany Key
  User hasMany Key
  </code></pre>
  
  <p>Now I want to find all rooms a specific user has a key for.</p>
  
  <p>At the moment I get all keys that have a specific <code>user_id</code>.</p>
  
  <p>Then I get all rooms and filter out the ones that don't have one of those found keys. Which seems quite dumb, since all of that could be done with one SQL command.</p>
  
  <p>I could probably pluck out the <code>room_id</code> of every key and use them in the query for the rooms, but those are still two requests.</p>