Update field in another schema with post hook mongoose - mongodb

Question

I have that schemas:
var schema = new Schema({
number: {type: Number},
subtotal: {type: Float, required: true},
quantity: {type: Number, required: true},
prodprov: {type: Schema.Types.ObjectId, ref: 'ProdProv'},
order: {type: Schema.Types.ObjectId, ref: 'Order'}
});
module.exports = mongoose.model('OrderDetail', schema);
and
var schema = new Schema({
name: {type: String, required: true},
price: {type: Float, required: true},
idProvider: {type: Schema.Types.ObjectId, ref: 'Provider'},
idProduct: {type: Schema.Types.ObjectId, ref: 'Product'},
description: {type: String},
quantitySold: {type: Number}
});
module.exports = mongoose.model('ProdProv', schema);
I need to update field "quantitySold" when a post.save happens in order-detail, i've tried this:
schema.post('save', function(next){
//But this point I can't find the prodprov to update field
})
How I can do this?
Thank you.

Answer 1

Thank you #mjim, but that´s code not works. I've modify the code and works! this is my solution:
schema.post('save', function(doc){
var prod = ProdProv.findById(this.prodprov).exec().then((pp) => {
pp.quantitySold += this.quantity;
pp.save();
});
});

Answer 2

I would suggest:
var ProdProv = require('./ProdProv');
var schema = new Schema({
number: {type: Number},
subtotal: {type: Float, required: true},
quantity: {type: Number, required: true},
prodprov: {type: Schema.Types.ObjectId, ref: 'ProdProv'},
order: {type: Schema.Types.ObjectId, ref: 'Order'}
});
schema.post('save', async function(doc){
var prod = await ProdProv.findById(doc.prodprov);
prod.quantitySold = doc.quantity;
prod.save()
});
module.exports = mongoose.model('OrderDetail', schema);