title: $concat field with index in $map mongodb?
body: <p>I have following collection</p>
  
  <pre><code>{
      "_id" : ObjectId("5b16405a8832711234bcfae7"),
      "createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
      "firstName": "Bruce",
      "lastName": "Wayne"
  },
  {
      "_id" : ObjectId("5b16405a8832711234bcfae8"),
      "createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
      "firstName": "Clerk",
      "lastName": "Kent"
  },
  {
      "_id" : ObjectId("5b16405a8832711234bcfae9"),
      "createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
      "firstName": "Peter",
      "lastName": "Parker"
  }
  </code></pre>
  
  <p>I need to <code>$project</code> one more key index with <code>$concat</code> with 'INV-00' + index of the root element</p>
  
  <p>My output should be something like that</p>
  
  <pre><code>{
      "_id" : ObjectId("5b16405a8832711234bcfae7"),
      "createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
      "firstName": "Bruce",
      "lastName": "Wayne",
      "index": "INV-001"
  },
  {
      "_id" : ObjectId("5b16405a8832711234bcfae8"),
      "createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
      "firstName": "Clerk",
      "lastName": "Kent",
      "index": "INV-002"
  },
  {
      "_id" : ObjectId("5b16405a8832711234bcfae9"),
      "createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
      "firstName": "Peter",
      "lastName": "Parker",
      "index": "INV-003"
  }
  </code></pre>
  
  <p>and can I change <code>createdAt</code> format to this <strong><code>Thu Jan 18 2018</code></strong> using <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/dateToString/" rel="nofollow noreferrer"><code>$dateToString</code></a> or something else???</p>
  
  <p>Thanks in advance!!!</p>