How to Aggregate data from a set within a document - mongodb

Question

I have a document that's modeled like this:
{
_id: '1',
timeEntries: [
{ _id: '1',
hours: '1'
},
{ _id: '2',
hours: '2'
},
],
totalHours: 3
}
Right now, every time I add a timeEntry to the set of timeEntries in my document, I also increment the totalHours property by the hours in the added timeEntry.
Instead of incrementing every time I $addToSet, I want to be able to call a method on the model of my document (virtual field?) to get the total hours from the document's timeEntries.
How would I go about doing this?
Update
Additionally, my timeEntries are actually stored as an array of reference.
Example:
[ 5bcf5e53e452b800134787dd, 5bcf5f42e452b800134787de ]
And I need a way to populate the hours property inside of the virtual field
Schema:
const companyHourLogSchema = new Schema({
dateOpened: {
type: Date,
default: Date.now,
},
dateClosed: {
type: Date,
},
_id: {
type: mongoose.Schema.ObjectId,
auto: true,
},
company: {
type: mongoose.Schema.ObjectId,
ref: 'Company',
},
timeEntries: [{
type: mongoose.Schema.ObjectId,
ref: 'TimeEntry',
}],
title: {
type: String,
default: 'Current',
},
});

Answer 1

update
because the time entries are saved as references you need to use mongoose populate to get the entries first
Model.find(/*your condition here*/).populate('timeEntries');
to get the timeEntries populated with the log.
you can use virtuals as follows:
schema.virtual('totalHours').get(function () {
return this.timeEntries.reduce((agg,tiE) => agg + tiE.hours ,0);
});
then you can use it as follows:
console.log(modelInstance.totalHours);