unable to use $match operator for mongodb/mongoose aggregation with ObjectId

Question

Relatively Simple Scenario:

I have this Voucher object which has a user property (of type ObjectId). I want to get the sum of

Answer 1

var aggregate  = [
{
                $match: {
                    'lenderId': new mongoose.Types.ObjectId(req.user),
                    '_disbursed': true
                }
            },
            {
                $unwind:'$emi'
            },
            {
                $match: {
                    'emi._settled': false,
                }
            }
];