MongoDB query to change simple field into an object?

Fir this, you can use $rename. Let us first create a collection with documents −

> db.changeSimpleFieldDemo.insertOne({"StudentMarks":58,"StudentSubject":"MySQL"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e0847a825ddae1f53b62205")
}

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

> db.changeSimpleFieldDemo.find();

This will produce the following output −

{ "_id" : ObjectId("5e0847a825ddae1f53b62205"), "StudentMarks" : 58, "StudentSubject" : "MySQL" }

Here is the query to change a field into an object. The “obj” field is a temporary field name we have used below −

> db.changeSimpleFieldDemo.update({}, {$rename: {Student: 'obj'}}, {multi: true});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 })
> db.changeSimpleFieldDemo.update({},
... {$rename: {obj: 'Student.Marks', discountType: 'Student.Subject'}},
... {multi: true});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 })