The most hard part is to attach a subdocument into the document which hasn't created yet if we need the subdocument need to be in the expected looking we will need to iterate with a for loop the array into a variable and using $doc2.add("Key", "Value") instead using the foreach current array with index. This will make the subdocument in two lines as you can see in the "Tags" = [MongoDB.Bson.BsonDocument] $doc2.