Mongoose aggregate the property of subdocument and display the result - mongodb

Question

I have a document with a subdocument (not referenced). I want to apply the aggregation on the field of the subdocument.
Schema
const MFileSchema = new Schema({
path: String,
malwareNames: [String],
title: String,
severity: String // i want to aggregate bases on this field
});
const ScanSchema = new Schema({
agent: { type: Schema.Types.ObjectId, ref: "Agent" },
completedAt: Date,
startedAt: { type: Date, default: Date.now() },
mFiles: [MFileSchema] // array of malicious files schema
});
Model
let Scan = model("Scan", ScanSchema);
Task
Find the sum of severity in all scan documents of particular agents.
// agents is an array Agents (the schema is not important to show, consider the _id)
The Aggregation Query I am using
let c = await Scan.aggregate([
{ $match: { agent: agents } },
{ $project: { "mFiles.severity": true } },
{ $group: { _id: "$mFiles.severity", count: { $sum: 1 } } }
]);
console.log(c);
Actual Output
[]
Expected Output
// The value of count in this question is arbitrary
[
{ _id: "Critical", count: 30 },
{ _id: "Moderate", count: 33 },
{ _id: "Clean", count: 500 }
]
PS: Also I would appreciate if you could suggest me the best resources to learn MongoDB aggregations

Answer 1

You need to use $in query operator in the $match stage, and add $unwind stage before $group stage.
db.collection.aggregate([
{
$match: {
agent: {
$in: [
"5e2c98fc3d785252ce5b5693",
"5e2c98fc3d785252ce5b5694"
]
}
}
},
{
$project: {
"mFiles.severity": true
}
},
{
$unwind: "$mFiles"
},
{
$group: {
_id: "$mFiles.severity",
count: {
$sum: 1
}
}
}
])
Playground
Sample data:
[
{
"agent": "5e2c98fc3d785252ce5b5693",
"mFiles": [
{
"title": "t1",
"severity": "Critical"
},
{
"title": "t2",
"severity": "Critical"
},
{
"title": "t3",
"severity": "Moderate"
},
{
"title": "t4",
"severity": "Clean"
}
]
},
{
"agent": "5e2c98fc3d785252ce5b5694",
"mFiles": [
{
"title": "t5",
"severity": "Critical"
},
{
"title": "t6",
"severity": "Critical"
},
{
"title": "t7",
"severity": "Moderate"
}
]
}
]
Output:
[
{
"_id": "Moderate",
"count": 2
},
{
"_id": "Critical",
"count": 4
},
{
"_id": "Clean",
"count": 1
}
]
For mongoose integration:
//agents must be an array of objectIds like this
// [ObjectId("5e2c98fc3d785252ce5b5693"), ObjectId("5e2c98fc3d785252ce5b5694")]
//or ["5e2c98fc3d785252ce5b5693","5e2c98fc3d785252ce5b5694"]
const ObjectId = require("mongoose").Types.ObjectId;
let c = await Scan.aggregate([
{
$match: {
agent: {
$in: agents
}
}
},
{
$project: {
"mFiles.severity": true
}
},
{
$unwind: "$mFiles"
},
{
$group: {
_id: "$mFiles.severity",
count: {
$sum: 1
}
}
}
]);
Best place for learning mongodb aggregation is the official docs.