Mongodb how to reduce the array within the matching key and calculate avg - mongodb

Question

{
"_id" : {
"state" : "NY",
"st" : "value"
},
"List" : [
{
"id" : "21",
"score" : 18.75,
"name" : "PU"
},
{
"id" : "21",
"score" : 25.0,
"name" : "PU"
},
{
"id" : "23",
"score" : 25.0,
"name" : "CL"
},
{
"id" : "23",
"score" : 56.25,
"name" : "CL"
}
]
}
Desired result:
Match the key with id within the array and calculate avg of score.
{
"_id" : {
"state" : "New York",
"st" : "value"
},
"List" : [
{
"id" : "21",
"score" : 21.875,
"name" : "PU"
},
{
"id" : "23",
"score" : 40.625,
"name" : "CL"
}
]
}
Thank you in advance.

Answer 1

Query
(returns the expected result)
unwind List
group with including the id, and find avg
fix the structure to be similar with the document you want
group back to restore the document structure (reverse the unwind)
if 2 sames ids have different name(if possible to happen)
query will make them seperated members in the array.
(alternativly it could make them same member and pack the names in an array, but that would produce different schema from the one you expect to see)
Test code here
db.collection.aggregate([
{
"$unwind": {
"path": "$List"
}
},
{
"$group": {
"_id": {
"state": "$_id.state",
"st": "$_id.st",
"id": "$List.id",
"name": "$List.name"
},
"avg": {
"$avg": "$List.score"
}
}
},
{
"$project": {
"_id": {
"state": "$_id.state",
"st": "$_id.st"
},
"List": {
"name": "$_id.name",
"id": "$_id.id",
"avg": "$avg"
}
}
},
{
"$group": {
"_id": "$_id",
"List": {
"$push": "$List"
}
}
}
])