MongoDB count,min,max,avg for aggregate using field with List of objects - mongodb

Question

I have problem with get statistics data using MongoDB collection.
Collection
[
{
"_id": {"$oid": "616309f71b021c754992bfca"},
"correlatedNodes": [
{
"type": "IS_ULTIMATELY_CONSOLIDATED_BY",
"nodes": ["I07WOS4YJ0N7YRFE7309"]
}
],
"fromTaskId": 20,
"nodeName": "5493000U0YGG4VEQOX65"
},
{
"_id": {"$oid": "616309f71b021c754992bfcb"},
"correlatedNodes": [
{
"type": "IS_DIRECTLY_CONSOLIDATED_BY",
"nodes": ["529900P6OUCFPVWCFE19", "8945007IZBKFQUQLIP85"]
},
{
"type": "IS_ULTIMATELY_CONSOLIDATED_BY",
"nodes": ["529900P6OUCFPVWCFE19"]
}
],
"fromTaskId": 20,
"nodeName": "89450012XZ2GPWGIGH37"
},
{
"_id": {"$oid": "616309f71b021c754992bfcc"},
"correlatedNodes": [
{
"type": "IS_ULTIMATELY_CONSOLIDATED_BY",
"nodes": ["815600C9588783AB0210"]
},
{
"type": "IS_DIRECTLY_CONSOLIDATED_BY",
"nodes": ["815600C9588783AB0210"]
}
],
"fromTaskId": 20,
"nodeName": "815600228127946DFF05"
},
{
"_id": {"$oid": "616309f71b021c754992bfcd"},
"correlatedNodes": [
{
"type": "IS_ULTIMATELY_CONSOLIDATED_BY",
"nodes": ["549300LI58A0MHGHTZ98"]
}
],
"fromTaskId": 20,
"nodeName": "549300NV4OCF16TAS048"
}
]
I want to get basic statistic information for correlatedNodes field.
Something like:
{
"IS_ULTIMATELY_CONSOLIDATED_BY": {
"count": 567,
"avg": 22,
"min": 3,
"max": 50
}
}
By count I mean to use list size for each connection type.
I tried aggregate query like this:
db.node_correlations.aggregate([
{
$project: {"correlatedNodes": "$correlatedNodes" },
$unwind: "$correlatedNodes"
}
]);
But I received error:
[2021-10-13 12:14:10] com.mongodb.MongoCommandException: Command failed with error 40323 (Location40323): 'A pipeline stage specification object must contain exactly one field.' on server localhost:27017. The full response is {"ok": 0.0, "errmsg": "A pipeline stage specification object must contain exactly one field.", "code": 40323, "codeName": "Location40323"}

Answer 1

Thank you everyone for help.
Working solutions is:
db.node_correlations.aggregate([
{
"$unwind": "$correlatedNodes"
},
{
"$group": {
"_id": "$correlatedNodes.type",
"count": {
"$sum": {"$size": "$correlatedNodes.nodes"}
},
"avg": {
"$avg": {"$size": "$correlatedNodes.nodes"}
},
"min": {
"$min": {"$size": "$correlatedNodes.nodes"}
},
"max": {
"$max": {"$size": "$correlatedNodes.nodes"}
}
}
}
])