title: Retrieve random ID key generated by Firebase?
body: <p>How do I retrieve the random ID key generated by Firebase?</p>
  
  <p>For example, if I have this:</p>
  
  <pre><code>    users
      │
      ├──── KT4NTZTzFduj3DNLQgg
      │     │
      │     ├────  uid: "7ZLldIsRu3NQMOb6"
      │     └────  username: "John"
      │
      │
      └──── TO39dsjk2wREF34kmcs // I want this
            │
            ├────  uid: "455klfmckjsnenk2sxkm2"
            └────  username: "Micheal"
  </code></pre>
  
  <p>I want to get the key which holds a username with value equal to Micheal.</p>
  
  <pre><code>firebase.database()
      .ref('users')
      .orderByChild("username")
      .equalTo("Micheal")
      .once("value", 
              function (snapshot) {
                  // what should I write here ??
              });
  </code></pre>