Update in MongoDB and prevent overwrite?

Let us create a collection with documents −

> db.demo601.insertOne(
...    {
...       id:1,
...       userDetails:
...          {
...             userName:"John",
...             userMailId:"John@gmail.com"
...          }
...       }
...    );
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e95ff5ced011c280a0905c7")
}
>
> db.demo601.insertOne( { id:2, userDetails: { userName:"Carol",
userMailId:"Carol@gmail.com" } } );{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e95ff71ed011c280a0905c8")
}

Display all documents from a collection with the help of find() method −

> db.demo601.find();

This will produce the following output −

{ "_id" : ObjectId("5e95ff5ced011c280a0905c7"), "id" : 1, "userDetails" : { "userName" : "John", "userMailId" : "John@gmail.com" } }
{ "_id" : ObjectId("5e95ff71ed011c280a0905c8"), "id" : 2, "userDetails" : { "userName" : "Carol", "userMailId" : "Carol@gmail.com" } }

Following is the query to update −

>db.demo601.update({_id:ObjectId("5e95ff71ed011c280a0905c8")},{$set:{userMailId:"Carol@yahoo.com"}});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Display all documents from a collection with the help of find() method −

> db.demo601.find();

This will produce the following output −

{ "_id" : ObjectId("5e95ff5ced011c280a0905c7"), "id" : 1, "userDetails" :
   { "userName" : "John", "userMailId" : "John@gmail.com" }
}
{ "_id" : ObjectId("5e95ff71ed011c280a0905c8"), "id" : 2, "userDetails" :
   { "userName" : "Carol", "userMailId" : "Carol@gmail.com" }, "userMailId" : "Carol@yahoo.com" 
}