title: Error: Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]" (ionic 2 and firebase)
body: <pre><code>saveDetails(){
    this.afAuth.authState.take(1).subscribe(auth =&gt; {
      this.af.object('request/${auth.uid}').set(this.request)
      .then();
    })
  </code></pre>
  
  <p>After executing this method, an error: Firebase.child failed: First argument was an invalid path: "request/${auth.uid}". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]" is being shown. Tried removing $ but that doesn't work. Pretty sure i am not passing in empty strings too.</p>