title: What is the firestore equivalent of this.db.object?
body: <p>Using this concept / tutorial for a project (<a href="https://angularfirebase.com/lessons/custom-usernames-with-firebase-authentication-and-angular/" rel="nofollow noreferrer">https://angularfirebase.com/lessons/custom-usernames-with-firebase-authentication-and-angular/</a>) 
  However it is in firebase, I need the Firestore equivalent.</p>
  
  <p>I have tried the following:</p>
  
  <pre><code>  checkUsername(tag: string) {
      console.log('Checking Username: ' + username);
      return  this.afs.collection('usernames', ref =&gt; ref.where('username', '==', tag)).valueChanges();
    }
  </code></pre>
  
  <p>I would like to only retrieve that specific field in the collection and check if it is available or not.</p>
  
  <p>Thanks</p>