How to create nested array inside $group in MongoDB? - mongodb

Question

After some of the aggregation pipeline stages, my MongoDB Document has the following format:
{
"key": "A",
"status": "To Do",
"val": 4
},
{
"key": "A",
"status": "Done",
"val": 1
},
{
"key": "A",
"status": "To Do",
"val": 3
},
{
"key": "A",
"status": "Done",
"val": 2
},
{
"key": "B",
"status": "Done",
"val": 5
},
{
"key": "B",
"status": "Done",
"val": 6
}
Expected output:
Group by key
Group by status
Push val
{
"key": "A",
"status_val": [
{
"status": "To Do",
"val": [
3,
4
]
},
{
"status": "Done",
"val": [
1,
2
]
}
]
},
{
"key": "B",
"status_val": "Done",
"val": [
5,
6
]
}
Here is what I tried:
db.collection.aggregate([
{
$group: {
_id: {
key:"$key",
status:"$status"
},
v: {
$push: "$val"
}
}
},
{
$group: {
_id: "$_id.key",
status_val: {
$addToSet: {
status: "$_id.status",
val: "$_id.v"
}
}
}},
{
$project: { _id: 0,key:"$_id",status_val:1 }
}
]).pretty()
But I get the wrong output:
[
{
"key": "B",
"status_val": [
{
"status": "Done"
}
]
},
{
"key": "A",
"status_val": [
{
"status": "Done"
},
{
"status": "To Do"
}
]
}
]
How to get nested array inside $group?

Answer 1

You're correctly running $group twice but this line needs to be changed:
val: "$_id.v"
v is not a part of _id, your data after first $group looks like this:
{
"_id": { "key": "B, "status": "Done" },
"v": [ 5, 6 ]
}
try:
db.collection.aggregate([
{
$group: {
_id: { key: "$key", status: "$status" },
val: { $push: "$val" }
}
},
{
$group: {
_id: "$_id.key",
status_val: {
$push: {
status: "$_id.status",
val: "$val"
}
}
}
},
{
$project: {
_id: 0,
key: "$_id",
status_val: 1
}
}
])
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