title: How to find if a key exists in MongoDb document?
body: <p>I have the following mongodb document</p>
  
  <pre><code>{
  
  "user_100":{
      "name": "Scott" ,
      "uniqueDetails":{
      "mobile":"9999999999",
      "email": "scott@abc.com",
      }
  },
  "user_101":{
      "name": "Smith",
      "uniqueDetails":{
      "mobile":"9999999998",
      "email": "smith@abc.com"
      }
  }
  }
  </code></pre>
  
  <p>Now, when a new user signs up, I would like to check if a given mobile number/email already exists. Is there any solution for this with <strong>Java MongoDB API</strong>.</p>