Mongo group inside $addToSet - mongodb

Question

I have the following set of objects:
[
{
id: 1,
clientId: 1,
cost: 200
},
{
id: 1,
clientId: 2,
cost: 500
},
{
id: 1,
clientId: 2,
cost: 800
},
{
id: 2,
clientId: 1,
cost: 600
},
{
id: 2,
clientId: 2,
cost: 100
}
]
And I made a group of that with:
db.collection.aggregate(
{
'$group': {
'_id': '$id',
'clients': {
'$addToSet': {
'id': '$clientId',
'cost': '$cost'
}
}
}
}
)
So I obteined the following:
[
{
'_id': 1,
'clients': [
{
id: 1,
cost: 200
},
{
id: 2,
cost: 500
},
{
id: 2,
cost: 800
}
],
'_id': 2,
'clients': [
{
id: 1,
cost: 600
},
{
id: 2,
cost: 100
}
]
}
]
As you can see in the array of clients of the first value, I have 2 repeated and what I want is to have 1 with the cost added. So instead of have:
'clients': [
{
id: 1,
cost: 200
},
{
id: 2,
cost: 500
},
{
id: 2,
cost: 800
}
]
I need:
'clients': [
{
id: 1,
cost: 200
},
{
id: 2,
cost: 1300
}
]
So my question is: how can I do that? Because $addToSet nor $push allow $sum.

Answer 1

You can use aggregation operators to get expected output like following:
db.collection.aggregate({
"$group": {
"_id": {
"mainId": "$id",
"client": "$clientId"
},
"cost": {
"$sum": "$cost"
}
}
}, {
"$project": {
"mainId": "$_id.mainId",
"clients": {
"clientId": "$_id.client",
"cost": "$cost"
},
"_id": 0
}
}, {
"$group": {
"_id": "$mainId",
"clients": {
"$push": "$clients"
}
}
})