Second $project Stage Producing Unexpected Result in MongoDB Aggregation - mongodb

Question

I am trying to run some aggregation in my MongoDB backend where I calculate a value and then add that calculated value to another value. The first step is working, but the second step produces a value of null, and I'm trying to understand why, and how to fix it.
This is what my aggregation look like:
db.staff.aggregate({
$project: {
_id: 1,
"workloadSatisfactionScore": {
$cond: [ { $eq: [ "$workload.shiftAvg", 0 ] }, "N/A", { $divide: [ "$workload.shiftAvg", "$workload.weeklyShiftRequest.minimum" ] } ]
}
},
$project: {
_id: 1,
totalScore: {
$sum: [ "$workloadSatisfactionScore", 10 ]
},
}
})
Even though the first $project stage produces documents with a numeric result or null for 'workloadSatisfactionScore', after the second $project stage, ALL documents have a value of null for 'totalScore'.
What I should get is whatever the value of 'workloadSatisfactionScore' is, added to 10. But as I say, all I get is null for all documents. What looks incorrect here?
As an example, one particular document in my collection returns a value of 0.9166666666666666 for "workloadSatisfactionScore". So when that is plugged into the second $project stage I'd expect a value of 10.9166666666666666 for 'totalScore'. But, as I say, instead I get null for that document, and all other documents.

Answer 1

It's possible that by the time the second $project pipeline is reached, workloadSatisfactionScore could be a string i.e. with the value "N/A" which will result in null when $add or $sum is used with a non-numerical value.
No need for the second project pipeline, you can add the value in the other conditional which handles the non-numerical part without passing it down the pipeline:
db.staff.aggregate({
"$project": {
"_id": 1,
"totalScore": {
"$cond": [
{ "$eq": [ "$workload.shiftAvg", 0 ] },
"N/A",
{ "$add": [
10,
{ "$divide": [
"$workload.shiftAvg",
"$workload.weeklyShiftRequest.minimum"
] }
] }
]
}
}
})