Object to group id in MongoDB pipelines - mongodb

Question

I have a MongoDB aggregation pipeline that returns such an output:
[
{
"id": {
"isPaid": false,
"state": "approved",
"updatedAt": "2018-06-27"
},
"state": "approved",
"isPaid": false,
"updatedAt": "2018-06-27",
"totalCount": 1,
"totalValue": 658.4332
},
{
"id": {
"isPaid": false,
"state": "canceled",
"updatedAt": "2018-05-30"
},
"state": "canceled",
"isPaid": false,
"updatedAt": "2018-05-30",
"totalCount": 1,
"totalValue": 1735.7175
},
]
For the system that exploits it, I'd need id to be available, as a string.
So I'm wondering if, using an aggregation pipeline stage, there would be an elegant and generic way to concatenate/serialize object values to a string:
"id": {"isPaid": false, "state": "approved", "updatedAt": "2018-06-27"}
to something like:
"id": "0.approved.2018-06-27"

Answer 1

you can use $concat
db.t55.aggregate([
{$addFields : {
id : {$concat : [{$cond :["$isPaid", "1","0"]}, ".", "$state", "." ,"$updatedAt"]}
}}
])
using $toString if not just true/false
db.t55.aggregate([
{$addFields : {
id : {$concat : [{$toString :{$indexOfArray : [[false,true], "$isPaid"]}}, ".", "$state", "." ,"$updatedAt"]}
}}
])
output
{ "_id" : ObjectId("5c3e794b21526e3ff4bf4ca2"), "id" : "0.approved.2018-06-27", "state" : "approved", "isPaid" : false, "updatedAt" : "2018-06-27", "totalCount" : 1, "totalValue" : 658.4332 }
{ "_id" : ObjectId("5c3e794b21526e3ff4bf4ca3"), "id" : "0.canceled.2018-05-30", "state" : "canceled", "isPaid" : false, "updatedAt" : "2018-05-30", "totalCount" : 1, "totalValue" : 1735.7175 }