How to use aggregate and group by two fields - mongodb

Question

I'm trying to use aggregate group and match to get my data.
This is my code:
itemShell.aggregate(
[
{
$match: { shell_id_in_whareHouse: {$in:shelfIds}}
},
{
$group: {
_id: "$item",
position:{'$last':'$position'},
total: { $sum: "$amount" }
}
}
],
function(err, results) {
if (err) console.log(err)
else {
res.json(results);
}
}
);
This is how itemShell objects looks like:
{item:1313,position:'2A',amount:500},
{item:1313,position:'2A',amount:200},
{item:1414,position:'1A',amount:500},
{item:1414,position:'2A',amount:800},
{item:1313,position:'1A',amount:300}
My problem is that the outcome of results is: (because of $Last accumulator)
[
{_id:1313,position:'1A',total:1000},
{_id:1414,position:'2A',total:1300},
]
My desired outcome should be:
[
{_id:1313,position:'2A',total:700},
{_id:1414,position:'1A',total:500},
{_id:1414,position:'2A',total:800},
{_id:1313,position:'1A',total:300}
]
So it will group only the objects that item number and position string are the same.
any suggestions ?

Answer 1

You're on the right path, you need to group by two fields using $group stage with _id as an object like below:
ItemShell.aggregate([
{
$match: {}
},
{
$group: {
_id: { item: '$item', position: '$position' },
total: { $sum: '$amount' }
}
},
{
$project: {
_id: 0,
item: '$_id.item',
position: '$_id.position',
total: 1
}
}
]);
Output
[
{
"item": 1313,
"position": "1A",
"total": 300
},
{
"item": 1313,
"position": "2A",
"total": 700
},
{
"item": 1414,
"position": "2A",
"total": 800
},
{
"item": 1414,
"position": "1A",
"total": 500
}
]
Here's a mongo playground: https://mongoplayground.net/p/Ne5h9WkzjEm

Answer 2

I think this should work
itemShell.aggregate([
{
"$group": {
"_id": {
"item": "$item",
"positon": "$positon"
},
"items": {
"$first": "$$ROOT"
},
"amount": {
"$sum": "$amount"
}
}
},
{
"$project": {
"item": "$items.item",
"positon": "$items.position",
"amount": 1,
"_id": "$items._id"
}
}
],
function(err, results) {
if (err) console.log(err)
else {
res.json(results);
}
}
);
You can see the results here : https://mongoplayground.net/p/h7aa_so1Cok
Output:
[
{
"_id": ObjectId("5a934e000102030405000000"),
"amount": 1000,
"item": 1313,
"positon": "2A"
},
{
"_id": ObjectId("5a934e000102030405000002"),
"amount": 1300,
"item": 1414,
"positon": "1A"
}
]