title: How to get the Object key of the associated object in angularFire2?
body: <p>I query the database by userkey to find if an associated object exists:</p>
  
  <p><code>let url = ``/userMember/${userKey}``;</code></p>
  
  <pre><code>const userMemberRef = this.af.database.object(url, { preserveSnapshot: true });
  
  userMemberRef.subscribe(data =&gt; {
    if(data.val()!=null) {
      console.log(data.val());
      memberKey= data.val();
    }
  });
  return memberKey;
  </code></pre>
  
  <p>It logs this in console which is good because the data I want is there.</p>
  
  <p><code>Object {-Ke2CyV2BJ5S3_7qcQj5: true}</code></p>
  
  <p>But how can I return the child key value "<code>-Ke2CyV2BJ5S3_7qcQj5</code>"?</p>
  
  <p>Seems to be very trivial. I appreciate your help!</p>