mongodb update query to append a data to already existing field [duplicate]

For a web application form i'm trying to add data to mongo collection on click of submit. my mongo collection is as below :

collection1:

 { "_id" : ObjectId("56e0a3a2d59feaa43fba49d5"), "timestamp" : ISODate("2017-11-18T10:23:29.620Z"), "City" : "London ", "LDE" :  "LDE-1234, LDE-345, LDE-456" }
 { "_id" : ObjectId("56e0a3a2d59feaa43fba49d6"), "timestamp" : ISODate("2016-12-18T10:23:29.620Z"), "City" : "Berlin", "LDE" : "LDE-444, LDE-3445, LDE-456" }
 { "_id" : ObjectId("56e0a3a2d59feaa43fba49d7"), "timestamp" : ISODate("2016-12-18T10:23:29.620Z"), "City" : "Dublin", "LDE" : "LDE-444, LDE-3445, LDE-7899, LDE-0909" }

In my form.html i have 2 input (City and LDE) 2 possibilities of adding value to db collection1 would be

Same City but new LDE value , then validate if LDE is present and update the document from collection . New City , then insert it into collection1.

The Update query i'm trying is as below :

 db.collection1.update({"City": "Dublin"}, { '$set' : {"LDE": "LDE-4443"}}), 

its changing the collection to : { "_id" : ObjectId("56e0a3a2d59feaa43fba49d7"), "timestamp" : ISODate("2016-12-18T10:23:29.620Z"), "City" : "Dublin", "LDE" : "LDE-4443" }

what I want is :

 { "_id" : ObjectId("56e0a3a2d59feaa43fba49d7"), "timestamp" : ISODate("2016-12-18T10:23:29.620Z"), "City" : "Dublin", "LDE" : "LDE-444, LDE-3445, LDE-7899, LDE-0909, LDE-4443" }

Also, Can this be done in one query both insert and update depending on the City value (without for or if loop)