how to fix this aggregation issue in mongodb redact - mongodb

Question

i have people dataset in mongodb, where i want to get all people whom friends has id greater than 1. i tried using $redact aggregation pipeline but it is not working. here is sample data
{
"_id" : "5d7a16904d08c0c435e4255b",
"name" : {
"first" : "Roberta",
"last" : "Jackson"
},
"company" : "YURTURE",
"email" : "roberta.jackson#yurture.io",
"age":26,
"registered" : "Sunday, May 5, 2019 2:44 PM",
"latitude" : "36.56389",
"longitude" : "-72.518115",
"friends" : [
{
"id" : 0,
"name" : "Vicki Peck"
},
{
"id" : 1,
"name" : "Jeanie Boyd"
},
{
"id" : 2,
"name" : "Terra Curtis"
}
],
}
i tried using aggregation pipeline redact but i am not getting any data in friends list.
db.getCollection('test_redact').aggregate([ { $project:{
"name" : 1,
"friends" : 1,
"greeting" : 1,
}},
{
$redact:{
$cond: {
if: { $gte: [ "$friends.id", 1 ] },
then: "$$DESCEND",
else: "$$PRUNE"
}
}
}
])
here is sample output i am getting after executing aggregation
{
"_id" : "5d7a16904d08c0c435e4255b",
"age" : 26,
"friends" : [],
"greeting" : "Hello, Roberta! You have 9 unread messages."
}

Answer 1

The following query can get us the expected output:
db.getCollection("collection").find({"friends.id":{$gte:1}},{"name":1,"friends":1,"greeting":1}).pretty()
To get only those friends which has id greater than 1:
db.getCollection("collection").aggregate([
{
$match:{
"friends.id":{
$gte:1
}
}
},
{
$project:{
"name":1,
"friends":{
$filter:{
"input":"$friends",
"as":"friend",
"cond":{
$gte:["$$friend.id",1]
}
}
},
"greeting":1
}
}
]).pretty()
Using $redact:
db.getCollection("collection").aggregate([
{
$redact:{
$cond:[
{
$or:[
{
$gte:["$id",1]
},
{
$eq:["$id",undefined]
}
]
},
"$$DESCEND",
"$$PRUNE"
]
}
},
{
$project:{
"name":1,
"friends":1,
"greeting":1
}
}
]).pretty()