Mongo $projection, can you flat a sub-document array? - arrays

Question

I was wondering if there is a way to "flatten" though projection a nested sub-document array so I could use it to sum its entries based on type.
My document looks like this:
{
"order_id":12345,
"date":8/17/2019,
"payment":{
status:1,
transactions:[
{type: 1, amount:200},
{type: 2, amount:250},
{type: 3, amount:50},
{type: 4, amount:50},
]
}
}
I would like to see if you can "flatten" it to something like this using $project:
{
"order_id":12345,
"date":8/17/2019,
"status":1,
"type": 1,
"amount":200
},
{
"order_id":12345,
"date":8/17/2019,
"status":1,
"type": 2,
"amount":250
},
{
"order_id":12345,
"date":8/17/2019,
"status":1,
"type": 4,
"amount":50
},
{
"order_id":12345,
"date":8/17/2019,
"status":1,
"type": 4,
"amount":50
}
}
Primarily my goal is to aggregate all the amounts for transactions of type 1 & 3 and all the transactions with type 2 & 4.
Any help would be great.

Answer 1

The following query can get you the expected output:
db.check.aggregate([
{
$unwind:"$payment.transactions"
},
{
$project:{
"_id":0,
"order_id":1,
"date":1,
"status":"$payment.status",
"type":"$payment.transactions.type",
"amount":"$payment.transactions.amount"
}
}
]).pretty()
Output:
{
"order_id" : 12345,
"date" : "8/17/2019",
"status" : 1,
"type" : 1,
"amount" : 200
}
{
"order_id" : 12345,
"date" : "8/17/2019",
"status" : 1,
"type" : 2,
"amount" : 250
}
{
"order_id" : 12345,
"date" : "8/17/2019",
"status" : 1,
"type" : 3,
"amount" : 50
}
{
"order_id" : 12345,
"date" : "8/17/2019",
"status" : 1,
"type" : 4,
"amount" : 50
}