title: Missing or insufficient permissions firestore
body: <p>I'm getting error: Firebase missing or insufficient permissions</p>
  
  <p>Code :
    const demoCollectionRef = this.db.collection('items');
    return demoCollectionRef.add(item);</p>
  
  <p>I think this error is because of .add method of angularfire2.</p>
  
  <p>Firestore Rules:</p>
  
  <pre><code>function isAuthenticated() {
    return request.auth.uid != null;
  }
  
  function isAdmin() {
      return request.auth.token.admin == true;
  }
  
  allow create: if isAuthenticated();
  allow update, delete: if isAdmin(); // .add method checks update rules 
  // at the place of checking the create rules, which make it send error.
  </code></pre>
  
  <p>But if I change the update rule to:
  allow update, delete: if isAuthenticated(); it works fine</p>