Mongoose nested populate query

I'm trying to return a list of coupons that are associated with a specific userid and populate the _merchant references.

This query correctly populates the _merchant refs. If I could add:

where coupon.users.contains(myuserid) to my query that would get what I need

db.couponModel.find().populate('_merchant').exec(function(err, coupons) { 
    res.send(coupons);
});

Or this query finds the correct coupons that I need. If I could add:

populate(_merchant) to my query that would also get what I need.

db.userModel.findById(req.params.id).populate('coupons').exec(function(err, user) {
    res.send(user.coupons)
});

Schemas

var userSchema = new Schema({
    email: { type: String, required: true , unique: true },
    coupons: [{ type: Schema.ObjectId, ref: 'Coupon' }]
});

var couponSchema = new Schema({
    _merchant: { type: Schema.ObjectId, ref: 'Merchant', required: true },
    couponid: { type: Number, required: true, unique: true },
    users: [{ type: Schema.ObjectId, ref: 'User' }]


});

var merchantSchema = new Schema({
    name: { type: String, required: true , unique: true }
    coupons: [{ type: ObjectId, ref: 'Coupon' }],

});

I need some hybrid of these two queries to get what I want.