title: How to implement unique constraint on a child value of a node in Firebase?
body: <p>In my app I have to store students information having roll number attribute.I want to prevent users to enter duplicate roll number.So guide me how can I implement logic to ensure <strong>rollno</strong> to be unique. Image to reflect tree is as follows.</p>
  
  <p><a href="https://i.stack.imgur.com/v93Va.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/v93Va.png" alt="enter image description here"></a></p>