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" }