When I am update a record, it removes the existing fields and adds the new ones.

This was the record before the update:

```
{ "_id" : ObjectId("56a356863aa433ae37dc2cee"), "browser" : "Chrome", "version" : 47}

```

This was the command I executed:

```
db.collection('profiles')
  .update({ 
    '_id' : obj("56a356863aa433ae37dc2cee") }, 
    {"first_name" : first_name, "last_name" : last_name, "email" : email}, 
    function (err, result) {
      console.log(result);
  });

```

This was the record after I had executed the `update` command:

```
{ "_id" : ObjectId("56a356c9a08487ed3719e40a"), "first_name" : "kaushik", "last_name" : "makwana", "email" : "kdmakwana" }

```