How to take the duplicate records in mongodb - mongodb

Question

Here i have two document ,in this documents childNodes array ID is duplicate means , i want to take the userID and pedagogyID of the record,as per my documents second document under childNodes array 798 is coming duplicate, so i want to take the records
Documents
{
"userID" : "A",
"pedagogyID" : "100",
"summary" : {
"LearnProgress" : {
"childNodes" : [
{
"ID" : "123",
"status" : "in-progress"
},
{
"ID" : "456",
"status" : null
},
{
"ID" : "333",
"status" : null
}
],
}
}
}
{
"userID" : "B",
"pedagogyID" : "200",
"summary" : {
"LearnProgress" : {
"childNodes" : [
{
"ID" : "789",
"status" : "in-progress"
},
{
"ID" : "1010",
"status" : null
},
{
"ID" : "789",
"status" : null
}
],
}
}
}
Expected Output
{
"userID" : "B",
"pedagogyID" : "200",
}
MY Code
db.collectionname.aggregate(
[
{"$unwind":"$summary.LearnProgress.childNodes"},
{"$group":{
"_id":{"_id":"$_id","ID":"$summary.LearnProgress.childNodes.ID"},
"userID":{"$first":"$userID"},
"pedagogyID":{"$first":"$pedagogyID"},
"count":{"$sum":1}
}},
{"$match":{"count":{"$gt":1}}},
{"$group":{"_id":{"userID":"$userID","pedagogyID":"$pedagogyID"}}},
{"$replaceRoot":{"newRoot":"$_id"}}
],
{ allowDiskUse:true }
)

Answer 1

You can use below aggregation.
db.colname.aggregate([
{"$unwind":"$summary.LearnProgress.childNodes"},
{"$group":{
"_id":{"_id":"$_id","ID":"$summary.LearnProgress.childNodes.ID"},
"userID":{"$first":"$userID"},
"pedagogyID":{"$first":"$pedagogyID"},
"count":{"$sum":1}
}},
{"$match":{"count":{"$gt":1}}},
{"$group":{"_id":{"userID":"$userID","pedagogyID":"$pedagogyID"}}},
{"$replaceRoot":{"newRoot":"$_id"}}
],{"allowDiskUse":true})

Answer 2

db.collectionname.aggregate(
// Pipeline
[
// Stage 1
{
$unwind: {
path : "$summary.LearnProgress.childNodes",
}
},
// Stage 2
{
$group: {
_id:'$summary.LearnProgress.childNodes.ID',
count:{$sum:1},
pedagogyID:{$first:'$pedagogyID'},
userID:{$first:'$userID'}
}
},
// Stage 3
{
$match: {
count:{$gt:1}
}
},
// Stage 4
{
$project: {
userID:1,
pedagogyID:1,
_id:0
}
},
]
);