title: Check if value exists in firebase DB
body: <p>Is there a method in firebase, which can check if value exist in DB? Firebase has method <a href="https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#exists" rel="noreferrer">.exists()</a>, but according to docs it checks only the keys.</p>
  
  <p>I have the following structure:</p>
  
  <pre><code>{
    "users": {
      "-KKUmYgLYREWCnWeHCvO": {
        "fName": "Peter",
        "ID": "U1EL9SSUQ",
        "username": "peter01"
      },
      "-KKUmYgLYREWCnWeHCvO": {
        "fName": "John",
        "ID": "U1EL5623",
        "username": "john.doe"
      }
    }
  }
  </code></pre>
  
  <p>I want to check if ID with value <code>U1EL5623</code>exists.</p>