title: Adding .indexOn for nested data
body: <p>I am getting the firebase warning "Using an unspecified index. Consider adding ".indexOn": "E-mail" at /user/544dceb6-98d-4f7e/clients.</p>
  
  <p>The rule I added is:</p>
  
  <pre><code>{
    "rules": {
      "user": {
        "$clients": {
          ".indexOn": ["E-mail"]
        }
      }
    }
  }
  </code></pre>
  
  <p>This is the call I'm making:</p>
  
  <pre><code>firebaseRef.child(authData.uid + "/clients").orderByChild("E-mail").equalTo(this.props.params.clientId).on('value', function(clientSnapshot) { ... }
  </code></pre>
  
  <p>This is how my data is structured:</p>
  
  <pre><code>database
  + user
    + 544dceb6-98d-4f7e...
        + clients
            + -K96sXIXRUIOK8....
                + E-mail
  </code></pre>
  
  <p>I suspect the issue has something to do with the two layers of keys but can't find a solution.</p>
  
  <p>Many thanks!</p>