Query entities with count of related attributes in separate field in mongodb - mongodb

Question

I have two collections. boxes and balls. A ball can be in a box:
boxes:
[{
"_id": { "$oid": "box-a" },
"name": "Box A"
},{
"_id": { "$oid": "box-b" },
"name": "Box B"
}]
balls:
[{
"_id": { "$oid": "ball-a" },
"color": "red",
"boxId": { "$oid": "box-a" }
},{
"_id": { "$oid": "ball-b" },
"color": "green",
"boxId": { "$oid": "box-a" }
}]
Now I want to query all boxes with an additional field ballColors where I get an overview how many balls of what color are in the boxes:
[{
"_id": { "$oid": "box-a" },
"name": "Box A",
"ballColors": {
"red": 1,
"green": 1,
}
},{
"_id": { "$oid": "box-b" },
"name": "Box B",
"ballColors": {}
}]
I tried to solve it with an aggregation like following:
db.boxes.aggregate([
{$lookup: {
from: "balls",
localField: "_id",
foreignField: "boxId",
as: "ballColors"
}},
{$addFields: {
ballColors: "$ballColors.color"
}}
])
...but this gives me something like that:
[{
"_id": { "$oid": "box-a" },
"name": "Box A",
"ballColors": [
"red",
"green"
]
},{
"_id": { "$oid": "box-b" },
"name": "Box B",
"ballColors": []
}]
I also did some experiments with $unwind combined with $group but I have no clue how to get those information back into the original objects...
Is there a way to count the colors in ballColors and put it in an object? Or is there another better way to do this?

Answer 1

$lookup with pipeline, pass _id as boxID in let, $match boxId condition
$group by color and get total count
$project to show k as color and v as count
convert key-values array to object using $arrayTOobject in $addFields
db.boxes.aggregate([
{
$lookup: {
from: "balls",
let: { boxId: "$_id" },
pipeline: [
{ $match: { $expr: { $eq: ["$$boxId", "$boxId"] } } },
{
$group: {
_id: "$color",
count: { $sum: 1 }
}
},
{
$project: {
_id: 0,
k: "$_id",
v: "$count"
}
}
],
as: "ballColors"
}
},
{ $addFields: { ballColors: { $arrayToObject: "$ballColors" } } }
])
Playground