How to add key-value pair to object in MongoDB

How to add a new key:value pair to all existing objects of a mongoDB documents

Old Key and Value Pairs

> db.students.find().pretty();
 { "_id" : ObjectId("601594f5a22527655335415c"), "name" : "Doddanna" }                                                                                                  
 { "_id" : ObjectId("601594f5a22527655335415d"), "name" : "Chawan" }

Update New Key and Value Pairs Using updateMany() and $set

> db.students.updateMany({},{$set:{newKey1:"newValue1", newKey2:"newValue2", newKeyN:"newValueN"}});
 { "acknowledged" : true, "matchedCount" : 2, "modifiedCount" : 2 }

Have a look on Updated pretty result

> db.students.find().pretty();
  {
    "_id" : ObjectId("601594f5a22527655335415c"),
    "name" : "Doddanna",
    "newKey1" : "newValue1",
    "newKey2" : "newValue2",
    "newKeyN" : "newValueN"
   }
   {
    "_id" : ObjectId("601594f5a22527655335415d"),
    "name" : "Chawan",
    "newKey1" : "newValue1",
    "newKey2" : "newValue2",
    "newKeyN" : "newValueN"
    }

Just do something like that

db.foo.update({"_id" :ObjectId("...") },{$set : {"Monday.z":8}})