Preprocessing Mongoose documents right after querying - json

Question

Abstract
Hi, I have two models City and Country:
var Country = new Schema({
name: String,
population: String
});
var City = new Schema({
name: String,
timeZone: { type: Number, min: -12, max: 12 },
summerTime: Boolean,
country: { type: Schema.ObjectId, ref: 'Country'}
});
When I'm querying data from mongo using the following way:
function (request, response) {
var CityModel = mongoose.model('City', City);
CityModel.find().lean().populate('country').exec(function(err, docs){
response.send(JSON.stringify(docs));
});
}
But on client when I parse JSON, I get multiple instances of the same country:
var cities = $.get('/cities').then(function(citiesJson){
var cities = JSON.parse(citiesJson);
var washingtonIndex = ...;
var californiaIndex = ...;
var washington = cities[washingtonIndex];
var california = cities[californiaIndex];
washington.country.population = "300 000 000";
california.country.population = "500 000 000";
console.log([california.country.population, california.country.population]);
});
Resulting with two values ["300 000 000", "500 000 000"] in the console.
The problem
To prevent this kind of behaviour and preserve object references I'm doing JSON.decycle before object serialization:
...
response.send(JSON.stringify(JSON.decycle(docs)));
...
It works better than it should be. As a result I get the following JSON on the client:
// City:
{
name: "Bost",
deleted: "0",
country: {
$ref: "$[0]["city"]["country"]"
},
lastModified: "2013-08-06T23:44:11.000Z",
_id: {
_bsontype: "ObjectID",
id: "Rm1'"
},
approved: "1"
}
Notice the _id field which is getting serialized by reference so I don't get the actual ObjectId's string representation on the client, instead I'm getting some internal mongo representation of the id which is not usable on the client.
The question
1) Is there a way to setup per model or per schema preprocessing for all queried documents so that I convert ObjectId's into string
2) Probably there is some other more efficient way of dealing with this problem?
Thanks