Aggregate based on Array value - mongodb

Question

I have collection schema like :
{
"_id" : ObjectId("582ee289618a504e5c830e03"),
"updatedAt" : ISODate("2016-11-24T05:01:59.470Z"),
"createdAt" : ISODate("2016-11-18T11:14:17.912Z"),
"requestId" : "IER5R2H",
"pickupDetails" : {
"_id" : ObjectId("58367447b0a1ada74ad7af7e"),
"itemsCount" : 1,
"pickupItems" : [
{
"name" : "Newspaper",
"quantity" : 15,
"unit" : "kg",
"unitPrice" : 9,
"amount" : 135,
"_id" : ObjectId("58367447b0a1ada74ad7af7f")
}
]
}}
{
"_id" : ObjectId("58fff31a618a504e5c831191"),
"updatedAt" : ISODate("2016-11-21T13:37:51.267Z"),
"createdAt" : ISODate("2016-11-19T06:37:14.857Z"),
"requestId" : "M7OZY9O",
"pickupDetails" : {
"_id" : ObjectId("5832f8afb8ec77fa3c518f97"),
"itemsCount" : 2,
"pickupItems" : [
{
"name" : "Newspaper",
"quantity" : 18,
"unit" : "kg",
"unitPrice" : 11,
"amount" : 198,
"_id" : ObjectId("5832f8afb8ec77fa3c518f98")
},
{
"name" : "Plastic",
"quantity" : 4,
"unit" : "kg",
"unitPrice" : 11,
"amount" : 44,
"_id" : ObjectId("584a853e46c71be3585bfb5a")
}
]
}}
I need to add the quantity based on the name of pickupItems, like "Newspaper" etc. Suppose quantity of Newspaper for a particular day.
As per the below data, result should be like, {"Newspaper":33}, for a particular date.

Answer 1

try this :
db.collection.aggregate([
{
$unwind:"$pickupDetails.pickupItems"
},
{
$group:{
_id:"$pickupDetails.pickupItems.name",
quantity:{
$sum:"$pickupDetails.pickupItems.quantity"
}
}
}
])
output:
{ "_id" : "Plastic", "quantity" : 4 }
{ "_id" : "Newspaper", "quantity" : 33 }
you can add a $match stage at the begining of the pipeline to get results for a particular day