Parse json to mongose schema

My mongoose schema

const BillSchema = new Schema({
    fromDate: { type: Date, default: Date.now },
    toDate: { type: Date, default: Date.now },
    phoneNumber: { type: String },
    created: { type: Date},
    user: { type: Schema.ObjectId, ref: 'User' },
    billDetail: [{ type: Schema.Types.ObjectId, ref: 'BillDetail' }]
});
const BilldetailSchema = new Schema({
    amount: { type: Number },
    item: { type: Schema.ObjectId, ref: 'Item' },
    created: { type: Date, default: Date.now },
    user: { type: Schema.ObjectId, ref: 'User' }
});
const ItemSchema = new Schema({
    name: { type: String},
    code: { type: String},
    amount: { type: Number },
    created: { type: Date, default: Date.now },
    user: { type: Schema.ObjectId, ref: 'User' }
});

My Json get from client

{ billDetail: [ { item: [Object], amount: 1, rentalPrice: 11, rentalDays: 1 } ],
  code: '000001',
  customerName: 'steve',
  phoneNumber: '959494949',
  fromDate: '2017-05-07T17:00:00.000Z',
  toDate: '2017-05-18T17:00:00.000Z' }

How can i parse json string above to bill object. i try use this code

let bill = new Bill(req.body);

but it's not working, it's only return a bill, but not billdetails

via Hiếu Giề