title: Firebase query if child of child contains a value
body: <p>The structure of the table is:</p>
  
  <ul>
  <li>chats</li>
  <li>--> randomId</li>
  <li>-->--> participants</li>
  <li>-->-->--> 0: 'name1'</li>
  <li>-->-->--> 1: 'name2'</li>
  <li>-->--> chatItems</li>
  </ul>
  
  <p>etc</p>
  
  <p>What I am trying to do is query the chats table to find all the chats that hold a participant by a passed in username string.</p>
  
  <p>Here is what I have so far:</p>
  
  <pre><code> subscribeChats(username: string) {
      return this.af.database.list('chats', {
          query: {
              orderByChild: 'participants',
              equalTo: username, // How to check if participants contain username
          }
      });
   }
  </code></pre>