title: Multiple Firestore queries, single promise / callback
body: <p>I understand that Firestore doesn’t support logical OR for queries.
  My idea is to create multiple queries and merge the results at the client.
  I am developing a news app and I am trying to fetch all articles that contain my users’ interests tags (ex. technology, music etc) 
  An average user has 20 tags so I will be making 20 different requests.</p>
  
  <p>Does anyone has experience with chaining multiple requests and returning a unique promise when all results arrive.? </p>
  
  <p>I am using the js sdk </p>
  
  <p>My data structure:</p>
  
  <p>articles (collection)</p>
  
  <pre><code>-article (document)
  --id: 10
  --time: 1502144665
  --title: "test title"
  --text: "test text"
  --tags(obj) 
  ---technology: 1502144665,
  ---politics: 1502144665,
  ---sports: 1502144665
  </code></pre>
  
  <p>So I will need to create multiple db requests like the following.</p>
  
  <pre><code>user.tags = ["technology","politics","sports","architecture","business"];
  
  for (var i = 0; i &lt; user.tags.length; i++) {
    db.collection('articles').where(user.tags[i], '&gt;', 0).orderBy(user.tags[i]))
      .get()
      .then(() =&gt; {
          // ... push to article array
    });)
  }
  </code></pre>
  
  <p>I am trying to figure out how to create a promise / callback when every request finishes.</p>