Use Aggregate with $group in mongodb - node.js

Question

I have data in worksheets collection like below:
/* 1 */
{
"_id" : ObjectId("5c21d780f82aa31334ab6506"),
"isBilling" : true,
"hours" : 6,
"userId" : ObjectId("5c1f38a1d7537d1444738493"),
}
/* 2 */
{
"_id" : ObjectId("5c21d780f82aa31334ab6507"),
"isBilling" : true,
"hours" : 4,
"userId" : ObjectId("5c1f38a1d7537d1444738493"),
}
/* 3 */
{
"_id" : ObjectId("5c21e10fae07cc1204a5b647"),
"isBilling" : false,
"hours" : 8,
"userId" : ObjectId("5c1f388fd7537d1444738492"),
}
I have to create a aggregate query to sum the hours, where isBilling equals to true, and where isBilling equals to false.I want the below output:
{
"billingHours":10,
"fixContract":8
}
I have to get data with the particular userId. I tried the below:
Worksheet.aggregate([
{
$match: conditions
},
{
$lookup:{
"from": "worksheets",
"let": {},
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$isBilling",false] } } },
{
"$group": { "_id": null, "totalHours": { "$sum": "$hours" } }
},
],
"as": "billingHours"
}
},
{
"$project":{"billingHours":1}
}
])
I am getting the below result:
[
{
"_id": "5c21d780f82aa31334ab6506",
"billingHours": [
{
"_id": null,
"totalHours": 16
}
]
},
{
"_id": "5c21d780f82aa31334ab6507",
"billingHours": [
{
"_id": null,
"totalHours": 16
}
]
}
]
I don't know why it is giving me 16 hours instead of 10 and giving me two objects instead of 1.

Answer 1

You don't need to use $lookup here. Simple $group with $cond will do the job.
db.collection.aggregate([
{ "$group": {
"_id": null,
"billingHours": {
"$sum": {
"$cond": [{ "$eq": ["$isBilling", true] }, "$hours", 0]
}
},
"fixContract": {
"$sum": {
"$cond": [{ "$eq": ["$isBilling", true] }, 0, "$hours"]
}
}
}}
])