Mongodb Query to get the nth document - mongodb

Question

I need to create a query in mongodb that needs to return the SECOND TO THE LAST document. I am planning to use $group for this query but i dont know what aggregation function to use. I only know $first and $last.
I have an example collection below and also include the expected output. Thank you!
"_id" : ObjectId("60dc27ac54b7c46bfa1b84b4"),
"auditlogs" : [
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84be"),
"userid" : ObjectId("5ffe702d59a9205db81fcb69"),
"action" : "ADDTRANSACTION"
},
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84bd"),
"userid" : ObjectId("5ffe644f9493e05db9245192"),
"action" : "EDITPROFILE"
},
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84bc"),
"userid" : ObjectId("5ffe64949493e05db9245197"),
"action" : "DELETETRANSACTION"
} ]
"_id" : ObjectId("60dc27ac54b7c46bfa1b75ge2"),
"auditlogs" : [
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84bb"),
"userid" : ObjectId("5ffe64b69493e05db924519b"),
"action" : "ADDTRANSACTION"
},
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84ba"),
"userid" : ObjectId("5ffe65419493e05db92451d4"),
"action" : "ADDTRANSACTION"
},
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84b9"),
"userid" : ObjectId("5ffe65689493e05db92451d9"),
"action" : "CHANGEACCESS"
},
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84b8"),
"userid" : ObjectId("5ffe65819493e05db92451dd"),
"action" : "DELETETRANSACTION"
},
{
"_id" : ObjectId("60dc27ac54b7c46bfa1b84b7"),
"userid" : ObjectId("5ffe65df9493e05db92451f3"),
"action" : "EDITPROFILE",
]
OUTPUT:
{"_id" : ObjectId("60dc27ac54b7c46bfa1b84b4"),"_id" : ObjectId("60dc27ac54b7c46bfa1b84bd"),"userid" : ObjectId("5ffe644f9493e05db9245192"),"action" : "EDITPROFILE"},
{"_id" : ObjectId("60dc27ac54b7c46bfa1b75ge2"),"_id" : ObjectId("60dc27ac54b7c46bfa1b84b8"),"userid" : ObjectId("5ffe65819493e05db92451dd"),"action" : "DELETETRANSACTION"}

Answer 1

You can't have two _id keys in one single object.
I've made the parent object's id to _parentId you can give it's a name anything you want except _id
Aggregation:
db.collection.aggregate([
{
$unwind: "$auditlogs"
},
{
"$project": {
"_parentId": "$_id",
"_id": "$auditlogs._id",
"action": "$auditlogs.action",
"userid": "$auditlogs.userid",
}
}
])
Playground

Answer 2

You can slice the array by -2 to get the last two item, then by 1 to get first one. Therefore, the array will be left the second to the last. Finally, unwind auditlogs so it can be changed from array to object which is structure that you want.
db.collection.aggregate([
{
$project: { auditlogs : { $slice: [ "$auditlogs", -2 ] } }
},
{
$project: { auditlogs : { $slice: [ "$auditlogs", 1 ] } }
},
{
$unwind: "$auditlogs"
}
])