Add field and groupby in in MongoDB - mongodb

Question

I want to be able to add a new field in aggregation and return the average value of all records with the same session ID. Something like this, but it's not possible to use a groupby inside addfield:
{
$addFields: {
sessionData: {
$group: {
_id: {
"sessionId": "$sessionId"
},
avgScrollDepth: { $avg: "$scrollDepthChange" },
totalSessionLength: { $max: "$scrollDepthChange" }
}
}
}
},
Edit:
Lets say these are the documents:
{
"sessionId": 1,
"sessionDepth": 1
},
{
"sessionId": 1,
"sessionDepth": 2
},
{
"sessionId": 1,
"sessionDepth": 3
}
I would want to return documents like this:
{
"sessionId": 1,
"sessionDepth": 1,
"totalSessionLength": 3
},
{
"sessionId": 1,
"sessionDepth": 2,
"totalSessionLength": 3
},
{
"sessionId": 1,
"sessionDepth": 3,
"totalSessionLength": 3
}

Answer 1

You can do it this way:
We store inside tmp variable all grouped documents
Count totalSessionLength
Flatten tmp variable with $unwind operator
Add totalSessionLength into final result
db.collection.aggregate([
{
$group: {
_id: "$sessionId",
totalSessionLength: {
$sum: 1
},
tmp: {
$push: {
"sessionId": "$sessionId",
"sessionDepth": "$sessionDepth"
}
}
}
},
{
$unwind: "$tmp"
},
{
$project: {
"_id": 0,
"sessionId": "$tmp.sessionId",
"sessionDepth": "$tmp.sessionDepth",
"totalSessionLength": "$totalSessionLength"
}
}
])
MongoPlayground
Generic way:
db.collection.aggregate([
{
$group: {
_id: "$sessionId",
totalSessionLength: {
$sum: 1
},
tmp: {
$push: "$$ROOT"
}
}
},
{
$unwind: "$tmp"
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$tmp",
{
totalSessionLength: "$totalSessionLength"
}
]
}
}
}
])
MongoPlayground