Replace value with a string literal during MongoDB aggregation operation

Use MongoDB $literal to set a string literal. Let us first create a collection with documents −

>db.replacevaluedemo.insertOne({"StudentName":"Chris","StudentFavouriteSubject":{"TeacherName":"Bob","SubjectCode":"MySQL111"}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e0390a3f5e889d7a51994fd") } >db.replacevaluedemo.insertOne({"StudentName":"Mike","StudentFavouriteSubject":{"TeacherName":"David","SubjectCode":"3221Java"}}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e0390b8f5e889d7a51994fe") }

Following is the query to display all documents from a collection with the help of find() method −

> db.replacevaluedemo.find().pretty();

This will produce the following output −

{    "_id" : ObjectId("5e0390a3f5e889d7a51994fd"),    "StudentName" : "Chris",    "StudentFavouriteSubject" : {       "TeacherName" : "Bob",       "SubjectCode" : "MySQL111"    } } {    "_id" : ObjectId("5e0390b8f5e889d7a51994fe"),    "StudentName" : "Mike",    "StudentFavouriteSubject" : {       "TeacherName" : "David",       "SubjectCode" : "3221Java"    } }

Here is the query to replace value with a string literal −

> db.replacevaluedemo.aggregate([{ ...    "$project": { ...       "_id": 1, ...       "StudentFavouriteSubject": { ...          SubjectCode: 1, ...          TeacherName: { $literal: "UNKNOWN NAME" } ...       } ...    } ... }]).pretty();

This will produce the following output −

{    "_id" : ObjectId("5e0390a3f5e889d7a51994fd"),    "StudentFavouriteSubject" : {       "SubjectCode" : "MySQL111",       "TeacherName" : "UNKNOWN NAME"    } } {    "_id" : ObjectId("5e0390b8f5e889d7a51994fe"),    "StudentFavouriteSubject" : {       "SubjectCode" : "3221Java",       "TeacherName" : "UNKNOWN NAME"    } }