Query Mongodb Sum of firsts element of an array of objects - arrays

Question

I would like to write a query for summing each field payment of the first object inside an array, for each element of my database.
The schema is the following:
var schema = new Schema({
plate : String,
category : String,
brand : String,
model : String,
sign : String,
tax : [{
date : { type: Date, default: Date.now },
payment : { type: Number, default: 0 },
}],
});
I wrote the following function for my query:
function(callback){
Machine.aggregate(
[
{$unwind: "$tax"},
{$group : {
_id : null ,
tot : { $sum: "$tax.payment"}
}}
]
,callback);
}
But in this way I retrieve the sum of all the payments inside the array tax. My goal is to take only the first, so I tried with $tax.0.payment and using arrayElemAt : [$tax,0] but all my trials gave a tot = 0.

Answer 1

The idea here is pick out the first element of each of payment array field via $arrayElemAt with projection and then group-sum the field $group $sum.
Query:
db.collection.aggregate([
{
$project: {
firstPayment: {
$arrayElemAt: [
"$tax",
0
]
}
}
},
{
$group: {
_id: null,
PaymentSum: {
$sum: "$firstPayment.payment"
}
}
}
]);
Demo O/P:
[
{
"PaymentSum": 11,
"_id": null
}
]

Answer 2

Machine.aggregate({$unwind:
{path: "$tax"}
},
{$group:{
_id: "$_id",
payment: {$first: "$tax.payment"}
}},
{$group: {
_id: null,
total: {$sum: "$payment"}
}}
)
Explanation:
First I used $unwind on tax, then in the first $group stage I grouped them according to _id,
that way I will get the first payment information from unwinded tax array.
Then I used $sum to add them in the second $group stage.
I tested with this data:
Machine collection docs:
{
"_id" : ObjectId("5dbf09a4d7912bcbc61ee9e4"),
"tax" : [
{
"payment" : 10
},
{
"payment" : 20
}
]
},
{
"_id" : ObjectId("5dbf09aad7912bcbc61ee9e5"),
"tax" : [
{
"payment" : 30
},
{
"payment" : 40
}
]
},
{
"_id" : ObjectId("5dbf09afd7912bcbc61ee9e6"),
"tax" : [
{
"payment" : 50
},
{
"payment" : 60
}
]
}
The result I got is:
{ "_id" : null, "tot" : 90 }
I hope this fulfills your requirements.

Answer 3

Add $arrayElemAt in your aggregate like this..
Machine.aggregate(
[
{$unwind: "$tax"},
{$group : {
_id : null ,
tot : { $sum: { $arrayElemAt : [ "$tax.payment", 0 ]}
}}
]
,callback);