Convert a field to an array using MongoDB update operation?

To convert a field to an array, use $set operator. Let us first create a collection with documents −

> db.convertAFieldToAnArrayDemo.insertOne({"StudentSubject":"MongoDB"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5ce92d7778f00858fb12e91d") }

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

> db.convertAFieldToAnArrayDemo.find();

This will produce the following output −

{ "_id" : ObjectId("5ce92d7778f00858fb12e91d"), "StudentSubject" : "MongoDB" }

Following is the query to convert a field to an array using update operation with $set:−

> db.convertAFieldToAnArrayDemo.find().forEach(function(myDocument) {    db.convertAFieldToAnArrayDemo.update(       { _id: myDocument._id },       { "$set": { "StudentSubject": [myDocument.StudentSubject] } }    ); })

Let us check the document once again −

> db.convertAFieldToAnArrayDemo.find();

This will produce the following output −

{ "_id" : ObjectId("5ce92d7778f00858fb12e91d"), "StudentSubject" : [ "MongoDB" ] }