Mongoose Summing Up subdocument array elements having same alias - mongodb

Question

I have a document like this:
_id:'someId',
sales:
[
{
_id:'111',
alias:'xxx',
amount:500,
name: Apple, //items with same alias always have same name and quantity
quantity:2
},
{
_id:'222',
alias:'abc',
amount:100,
name: Orange,
quantity:14
},
{
_id:'333',
alias:'xxx',
amount:300,
name: Apple, //items with same alias always have same name and quantity
quantity:2
}
]
The alias field is here to 'group' items/documents whenever they appear to have same alias i.e to be 'embeded' as one with the amount summed up.
I need to display some sort of a report in such a way that those elements which have same alias they should be displayed as ONE and the others which doesn't share same alias to remain as they are.
Example, For the sample document above, I need an output like this
[
{
alias:'xxx',
amount:800
},
{
alias:'abc',
amount:100
}
]
WHAT I HAVE TRIED
MyShop.aggregate([
{$group:{
_id: "$_id",
sales:{$last :"$sales"}
},
{$project:{
"sales.amount":1
}}
}
])
This just displays as a 'list' regardless of the alias. How do I achieve summing up amount based on the alias?

Answer 1

You can achieve this using $group
db.collection.aggregate([
{
$unwind: "$sales"
},
{
$group: {
_id: {
_id: "$_id",
alias: "$sales.alias"
},
sales: {
$first: "$sales"
},
_idsInvolved: {
$push: "$sales._id"
},
amount: {
$sum: "$sales.amount"
}
}
},
{
$group: {
_id: "$_id._id",
sales: {
$push: {
$mergeObjects: [
"$sales",
{
alias: "$_id.alias",
amount: "$amount",
_idsInvolved: "$_idsInvolved"
}
]
}
}
}
}
])
Mongo Playground

Answer 2

You can use below aggregation
db.collection.aggregate([
{
"$addFields": {
"sales": {
"$map": {
"input": {
"$setUnion": [
"$sales.alias"
]
},
"as": "m",
"in": {
"$let": {
"vars": {
"a": {
"$filter": {
"input": "$sales",
"as": "d",
"cond": {
"$eq": [
"$$d.alias",
"$$m"
]
}
}
}
},
"in": {
"amount": {
"$sum": "$$a.amount"
},
"alias": "$$m",
"_idsInvolved": "$$a._id"
}
}
}
}
}
}
}
])
MongoPlayground