Update MongoDB variable value with variable itself?

You cannot update a column value using itself. For this, you can use $set. Let us create a collection with documents −

> db.demo256.insertOne({"Name":"Chris"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e47a3e91627c0c63e7dba8b")
}
> db.demo256.insertOne({"Name":"Bob"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e47a3ea1627c0c63e7dba8c")
}
> db.demo256.insertOne({"Name":"David"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e47a3eb1627c0c63e7dba8d")
}

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

> db.demo256.find();

This will produce the following output −

{ "_id" : ObjectId("5e47a3e91627c0c63e7dba8b"), "Name" : "Chris" }
{ "_id" : ObjectId("5e47a3ea1627c0c63e7dba8c"), "Name" : "Bob" }
{ "_id" : ObjectId("5e47a3eb1627c0c63e7dba8d"), "Name" : "David" }

Following is the query to update variable value with variable itself −

> db.demo256.update({}, {$set:{"Name":"Name" + " is a student"}});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

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

> db.demo256.find();

This will produce the following output −

{ "_id" : ObjectId("5e47a5301627c0c63e7dba8e"), "Name" : "Name is a student" }
{ "_id" : ObjectId("5e47a5311627c0c63e7dba8f"), "Name" : "Bob" }
{ "_id" : ObjectId("5e47a5311627c0c63e7dba90"), "Name" : "David" }