title: Get only collection IDs from Cloud Firestore in one read count
body: <p>In <a href="https://firebase.google.com/docs/firestore/pricing" rel="noreferrer">this</a> document of Cloud Foreshore it is mentioned that 'a request for a list of collection IDs, you are billed for one document read.'</p>
  
  <p>Now I am using AngularFire and Ionic3 for my project. My requirement is to fetch <strong>just the collection IDs</strong> from a collection with a where condition. If I do the following, it will cost me the number of reads = number of docs in the result.</p>
  
  <pre><code>let snap = this.afs.collection('path').ref
          .where('field', "==",'data').get();
  
  snap.forEach(x =&gt; {
            list.push(x.id);
          });
  </code></pre>
  
  <p>I am not able to use method <code>getCollections()</code> which I found in a few places as a solution.</p>
  
  <p>Please let me know if you have a solution.</p>