How can I get a sum of sums using MongoDB aggregation? - mongodb

Question

I would like to get the total (sum) of the totalHoursForYear value. Here is my current aggregation:
const byYear = await this.workHistoryModel.aggregate([
{
$group: {
_id: '$year',
history: {
$push: '$$ROOT'
},
totalHoursForYear: {
$sum: {
$add: [
{ $multiply: ['$eightHourDays', 8] },
{ $multiply: ['$tenHourDays', 10] },
{ $multiply: ['$twelveHourDays', 12] },
]
}
},
}
},
]);
Now I need to get a sum of the totalHoursForYear to get the total for all years. Is this possible? I can't seem to get the syntax correct. When I attempt to add another sum:
$group: {
...,
totalHours: {
$sum: {
$add: '$totalHoursForYear'
},
$push: '$$ROOT'
}
}
I get an error: "The field 'totalHours' must specify one accumulator"

Answer 1

When you need total sum, you can do another $group with _id:null which means to consider all documents
With your script, add the following to get the total and get the same structure.
{
$group: {
_id: null,
data: {
$push: "$$ROOT"
},
total: {
$sum: "$totalHoursForYear"
}
}
},
{
"$unwind": "$data"
},
{
"$addFields": {
_id: "$data._id",
history: "$data.history",
totalHoursForYear: "$data.totalHoursForYear",
_id: "$$REMOVE",
data: "$$REMOVE"
}
}
Working Mongo playground