Perform aggregation in an array of embedded doc - mongodb

Question

"employees":[
{
"empId":100,
"Salary":[
1000,2000,3000
]
},
{
"empId":101,
"Salary":[
3000,4000,500
]
}
]
In the above array, I need to aggregate the salary like 1000+3000,2000+4000 e.t.c and place it in the separate array.
My aggregated result should be :
adding salaries of both empId's
salary[0]+salray[0](empId:100 + empId:101=1000+3000,2000+4000,3000+500)
"employees":[
{
"empId":100,
"Salary":[
1000,2000,3000
]
},
{
"empId":101,
"Salary":[
3000,4000,500
]
},
{
"empId":111,
"Salary":[
4000,6000,3500
]
}
]

Answer 1

You need to $unwind all arrays and then group using aggregation framework
db.dev777.aggregate([{
$unwind : "$employees"
}, {
$unwind : "$employees.Salary"
}, {
$group : {
_id : "$employees.empId",
salarySum : {
$sum : "$employees.Salary"
}
}
}
])
OUTPUT:
{
"_id" : 101.0,
"salarySum" : 7500.0
},{
"_id" : 100.0,
"salarySum" : 6000.0
}
EDIT
db.dev777.aggregate([{
// transform array to document
$unwind : "$employees"
}, {
// transform array to document and add array index to preserve position info
$unwind : {
path : "$employees.Salary",
includeArrayIndex : "arrayIndex"
}
},
{
$group : {
// now sum all data by array index field
_id : "$arrayIndex",
salarySum : {
$sum : "$employees.Salary"
}
}
}, {
$sort : {
// sort by array index field
_id : 1
}
}, {
$group : {
// recreate document by pushing back values to an array
_id : null,
Salary : {
$push : "$salarySum"
}
}
}, {
$project : {
//remove id field and add empID field
_id : 0,
empID: {
$literal : NumberInt(111)
},
Salary : 1
}
}
])