title: How to get a keyname by its nested values in firebase?
body: <p><img src="https://i.stack.imgur.com/2z1YL.png" alt="enter image description here"></p>
  
  <p>My firebase is modeled this way. I want to write a query to get userID(simplelogin:1) where email is "test@gmail.com".</p>
  
  <p>Here I want to get key: "simplelogin:1" by searching with it's email address:"test@gmail.com"</p>
  
  <p>I am using firebase SDK for iOS. Can somebody suggest me a query for this? In Javascript or Objective-C</p>