Angular js - accessing nested objects arrays

Below is my schema.

**Payment.js**
var PaymentSchema = new Schema({   
    customer: {
       customerRef: {
            type: Schema.ObjectId,
            ref: 'Customer'
        },
        custMemberId : String,
        branch : String
       },
        paymentStatus : String,
        payBalance : Number,
        deliveryMode : String
});

**Order.js**
var OrderSchema = new Schema({
  OrderId : string,
  products :[{type : String}],
  totalCost : Number
});

**Customer.js**
var Customer= new Schema({
Name : String,
previousOrders :[{
      type: Schema.ObjectId,
            ref: 'Order']}

});

I have noted down some of the fields in the schema files. My problem is from a payment page,

If I refer to the previous orders of customer, I am not able to access the array previousOrders.

I try

console.log($scope.payment.customer.customerRef.previousOrders);

the output is 'undefined'

PaymentAPI - service

exports.payment = function(req, res, next, id) {
    Payment
        .findOne({
            _id: id
        })
        .populate('customer.customerRef')
        .exec(function(err, payment{
            if (err) return next(err);
            if (!payment) return next(new Error('Failed to load payment' + id));
                       req.payment = payment;

            next();
        });
};

Please let me know , the other possibilities.