Is there a way to add a counter to mongodb query? - mongodb

Question

I would like to add a counter to the documents that match my query. E.g., 1st document has counter = 1, 2nd document has counter = 2, and so on.
Here's a snippet of the data:
"_id": ObjectId("5d1b9aea5c1dd54e8c773f42")
"timestamp":
[
"systemTimestamp": 2019-07-02T17:56:53.765+00:00
"serverTimestamp": 0001-01-01T00:00:00.000+00:00
"systemTimeZone": "System.CurrentSystemTimeZone"
]
"urlData":
[0]:
"fullUrl":"https://imgur.com/gallery/EfaQnPY"
"UID":"00000-W3W6C42GWTRE960"
"safety": "safe"
My query (this is copied from the Compass UI):
$match:
{
$and: [{"UID": "00000-WVUCW3JW7OTHDVE"},
{"timestamp.serverTimestamp":
{
$gte:ISODate("2019-08-01T00:00"),
$lte:ISODate("2019-09-30T00:00")
}}]
}
$unwind:
{
path: "$urlData",
includeArrayIndex: 'index'
}
$match:
{
"index": 0
}
$project:
{
_id: 0,
date: { $dateToString: {
format: "%Y-%m-%d",
date: "$timestamp.serverTimestamp"}},
safety: "$safety",
url: "$urlData.fullUrl",
UID: "$UID"
}
Is there any way to add something to $project to include a counter?

Answer 1

The answer is there in your question itself. We can get the expected output if the output of the last pipeline is added into an array and again unwinded with the included index.
Let's say, I have the following data:
{
"_id" : ObjectId("5d81c3b7a832f81a9e02337b"),
"first" : "John",
"last" : "Smith"
}
{
"_id" : ObjectId("5d81c3b7a832f81a9e02337c"),
"first" : "Alice",
"last" : "Johnson"
}
{
"_id" : ObjectId("5d81c3b7a832f81a9e02337d"),
"first" : "Bob",
"last" : "Williams"
}
On running the following query:
db.collection.aggregate([
{
$group:{
"_id":null,
"data":{
$push:"$$ROOT"
}
}
},
{
$unwind:{
"path":"$data",
includeArrayIndex: 'counter'
}
},
{
$addFields:{
"data.counter":{
$sum:["$counter",1]
}
}
},
{
$replaceRoot:{
"newRoot":"$data"
}
}
]).pretty()
Output would be:
{
"_id" : ObjectId("5d81c3b7a832f81a9e02337b"),
"first" : "John",
"last" : "Smith",
"counter" : 1
}
{
"_id" : ObjectId("5d81c3b7a832f81a9e02337c"),
"first" : "Alice",
"last" : "Johnson",
"counter" : 2
}
{
"_id" : ObjectId("5d81c3b7a832f81a9e02337d"),
"first" : "Bob",
"last" : "Williams",
"counter" : 3
}