MongoDB count values of multible nested documents - mongodb

Question

My data looks something like that:
[
{
"_id": 1,
"members": [
{
"id": 1,
"name": "name_1",
"assigned_tasks": [
1,
2,
3
]
},
{
"id": 1,
"name": "name_2",
"assigned_tasks": [
1
]
}
],
"tasks": [
{
"id": 1,
"name": "task_1",
},
{
"id": 2,
"name": "task_2",
},
{
"id": 3,
"name": "task_3",
}
]
}
]
I have a collection that represents a "class" which contains a list of members and a list of projects.
Each member can be assigned to multiple projects.
I wanna be able to count the number of members assigned to each of the tasks in the results and add it as a new field like:
[
{
"_id": 1,
"members": [
{
"id": 1,
"name": "name_1",
"assigned_tasks": [
1,
2,
3
]
},
{
"id": 1,
"name": "name_2",
"assigned_tasks": [
1
]
}
],
"tasks": [
{
"id": 1,
"name": "task_1",
"number_of_assigned_members":2
},
{
"id": 2,
"name": "task_2",
"number_of_assigned_members":1
},
{
"id": 3,
"name": "task_3",
"number_of_assigned_members":2
}
]
}
]
How can I create that query?

Answer 1

You can use $map and than $reduce,
$map tasks through object by object check in $reduce on members, if assigned_tasks is available or not, if available then add 1 otherwise 0,
db.collection.aggregate([
{
$addFields: {
tasks: {
$map: {
input: "$tasks",
as: "t",
in: {
$mergeObjects: [
"$$t",
{
number_of_assigned_members: {
$reduce: {
input: "$members",
initialValue: 0,
in: {
$cond: [
{ $in: ["$$t.id", "$$this.assigned_tasks"] },
{ $add: ["$$value", 1] },
"$$value"
]
}
}
}
}
]
}
}
}
}
}
])
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