How to Join these two documents for API calls

[ghost]

City Document :
{
"_id" : ObjectId("555ec317e6e8ce4e0d3c2541"),
"state" : ObjectId("54f91ea29f2e20e67d61ba6c"),
"deleted" : false,
"created" : ISODate("2015-05-22T05:48:07.831Z"),
"description" : "",
"name" : "Hydrabad",
"__v" : 0
}

State Document:
{
"_id" : ObjectId("54e461ef7214e6583c0c5ed5"),
"__v" : 0,
"created" : ISODate("2015-02-18T09:57:03.585Z"),
"deleted" : false,
"description" : "",
"name" : "Andhra pradesh",
"updated" : ISODate("2016-10-09T18:20:29.601Z")
}

Schemas :
StateSchema = new Schema({
_id: { type: String },
statename: { type: String, required: true, max: 50 }

})

CitySchema = new Schema({
id: mongoose.Schema.Types.ObjectId,
name: { type: String, required: true, max: 50 },

statename: { type:String, ref: 'State' ,field: 'state'},
deleted: Boolean,

});

[gianpaj]

@Ramakrishnatp see http://mongoosejs.com/docs/populate.html