I have 2 collections, Country and cities, cities should hold the ObjectId reference of Countries,
On Creation of new city,
i want cities Collection should Look like
{
    "_id" : ObjectId("5c83505e5609a7274855ae4c"),
    "city_id" : "13456",
    "city_name" : "CapeTown",
    "country" :ObjectId("5c8350235609a7274855ae4b")
    "_class" : "com.mycompany.myapp.domain.cities"
}
I don't want something like
{
    "_id" : ObjectId("5c83505e5609a7274855ae4c"),
     "city_id" : "13456",
    "city_name" : "CapeTown",
    "country" : {
        "$ref" : "countries",
        "$id" : ObjectId("5c8350235609a7274855ae4b")
    },
    "_class" : "com.mycompany.myapp.domain.cities"
}
How to write the model class to achieve my requirement?
