The field must be an accumulator object in mongo [duplicate] - node.js

Question

This question already has answers here:
MongoDB SELECT COUNT GROUP BY
(9 answers)
Closed 3 years ago.
I am performing an aggregation query.My requirement is that I have docs in following format:
{
"name":"a","age":20},
{
"name":"b","age":23},
{
"name":"a","age":26}
I need to sum the ages of all names who have "a" in their name field.
I am doing the following but it gives me an error that "The field 'name' must be an accumulator object":
db.collection('students').aggregate({
$group: {"name": "a", "sum": {$sum: '$age'}}
}, {
$project: {sum: '$sum'}
}, (e, d) => {
if (!e) {
var e = d.stream({transform: JSON.stringify});
console.log("answer")
console.log(e);
deferred.resolve(e);
} else {
console.log(e)
}
})

Answer 1

I altered your aggregation query.Try as below:
db.collection('students').aggregate(
[
{
$match: {
name: "a"
}
},
{
$group: {_id: "$name",
"sum": {
$sum: '$age'
}
}
},
{
$project: {
sum: 1
}
], (e, d) => {
if (!e) {
var e = d.stream({transform: JSON.stringify});
console.log("answer")
console.log(e);
deferred.resolve(e);
} else {
console.log(e)
}
})

Answer 2

Here, you are missing match filter properly. We should use $match filter to find out specific results and "_id" for attribute on which we are performing operation in $group.
Structure of your query would be -
db.students.aggregate(
[
{
$match: {
"name": "a"
}
},
{
$group: {
"_id": "$name",
totalTte: {
$sum: "$age"
}
}
},
{
$project: {
sum: 1
}
}
])