how to perform group by in mongodb - node.js

Question

I'm having this data in MongoDB, I want to perform group by on this data to get all the tracking numbers, under the same id in a single array format.
* 1 */
{
"_id" : ObjectId("597056182a93692b4c7691bf"),
"Sid":1,
"Carriers" : [
{
"Tracking" : [
{
"TrackingNo" : "121_dom"
}
]
},
{
"Tracking" : [
{
"TrackingNo" : "779591314278"
},
{
"TrackingNo" : "779591314039"
},
{
"TrackingNo" : "779591314231"
}
]
}
]
}
/* 2 */
{
"_id" : ObjectId("597057338c65c002e4285fb3"),
"Sid":2,
"Carriers" : [
{
"Tracking" : [
{
"TrackingNo" : "122_dom"
}
]
},
{
"Tracking" : [
{
"TrackingNo" : "77959131427"
},
{
"TrackingNo" : "77959131403"
}
]
}
]
}
/* 3 */
{
"_id" : ObjectId("5980ae7ecc71b581b626d20b"),
"Sid":3,
"Carriers" : [
{
"Tracking" : [
{
"TrackingNo" : "123_dom"
}
]
},
{
"Tracking" : [
{
"TrackingNo" : "77959131408"
},
{
"TrackingNo" : "779591314059"
},
{
"TrackingNo" : "779591315551"
}
]
}
]
}
I.e on performing group by operation on _id, which shows all tracking numbers under the same _Id in a single array, means the result set will be like this :
{
"_id" : ObjectId("597056182a93692b4c7691bf"),
"Sid":1,
"Carriers" : [
{
"Tracking" : [
{
"TrackingNo" : "121_dom"
}
{
"TrackingNo" : "779591314278"
},
{
"TrackingNo" : "779591314039"
},
{
"TrackingNo" : "779591314231"
}
]
}
]
}

Answer 1

As you have array within array, you first need to unwind twice and then group again. Following query should work. You need to project additional fields in $group and $project as required.
db.shipments.aggregate(
{$unwind:'$Carriers'},
{$unwind: '$Carriers.Tracking'},
{$group: {_id:'$_id', c: {$push: '$Carriers.Tracking'}}},
{$project:{Carriers: {Tracking: '$c'}}}
)

Answer 2

According to description as mentioned in above question please try executing following aggregate query into MongoDB shell as a solution to above mentioned question.
db.shipments.aggregate(
// Pipeline
[
// Stage 1
{
$unwind: {
path: '$Carriers'
}
},
// Stage 2
{
$unwind: {
path: '$Carriers.Tracking'
}
},
// Stage 3
{
$group: {
_id: {
_id: '$_id',
Sid: '$Sid'
},
Carriers: {
$addToSet: {
TrackingNo: '$Carriers.Tracking.TrackingNo'
}
}
}
},
// Stage 4
{
$group: {
_id: '$_id',
Carriers: {
$addToSet: {
Tracking: '$Carriers'
}
}
}
},
// Stage 5
{
$project: {
_id: '$_id._id',
Sid: '$_id.Sid',
Carriers: '$Carriers'
}
}
]
);