Count nested and outer data - mongodb

Question

I have the following mongo data structure:
[
{
_id: "......",
libraryName: "a1",
stages: [
{
_id: '....',
type: 'b1',
},
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b3',
},
{
_id: '....',
type: 'b1',
},
],
},
{
_id: "......",
libraryName: "a1",
stages: [
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b1',
},
],
},
{
_id: "......",
libraryName: "a2",
stages: [
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b2',
},
{
_id: '....',
type: 'b1',
},
],
},
]
Assume this is the Session collection. Now, each session document has some irrelevant _id and libraryName key. Furthermore, each document has array of stages documents. Each stage document has some irrelevant _id and type. I want to count 2 things.
First - I want to count for each libraryName, how many session objects it has.
The solution for this query would be:
const services = await Session.aggregate(
[
{
$group: {
_id: "$libraryName",
count: { $sum: 1 },
},
}
]
);
Second - I want, per libaryName to count for each stage type how many nested stages documents it has.
So the final result I wish to retrieve is:
[
{
libraryName: 'a1',
count: 456,
stages: [
{
type: 'b1',
count: 43,
},
{
type: 'b2',
count: 44,
}
],
},
{
libraryName: 'a2',
count: 4546,
stages: [
{
type: 'b1',
count: 43
},
{
type: 'b3',
count: 44
}
]
}
]
Changed to:
[
{
"_id": "a1",
"count": 2,
"stages": [
{
"count": 1,
"type": "b3"
},
{
"count": 3,
"type": "b1"
},
{
"count": 4,
"type": "b2"
}
]
},
{
"_id": "a2",
"count": 1,
"stages": [
{
"count": 1,
"type": "b1"
},
{
"count": 3,
"type": "b2"
}
]
}
]

Answer 1

Using the sample data in the question post and the aggregation query:
db.collection.aggregate([
{
$unwind: "$stages"
},
{
$group: {
_id: { libraryName: "$libraryName", type: "$stages.type" },
type_count: { "$sum": 1 }
}
},
{
$group: {
_id: { libraryName: "$_id.libraryName" },
count: { "$sum": "$type_count" },
stages: { $push: { type: "$_id.type", count: "$type_count" } }
}
},
{
$project: {
libraryName: "$_id.libraryName",
count: 1,
stages: 1,
_id: 0
}
}
])
I get the following results:
{
"libraryName" : "a2",
"count" : 4,
"stages" : [
{
"type" : "b1",
"count" : 1
},
{
"type" : "b2",
"count" : 3
}
]
}
{
"libraryName" : "a1",
"count" : 8,
"stages" : [
{
"type" : "b3",
"count" : 1
},
{
"type" : "b1",
"count" : 3
},
{
"type" : "b2",
"count" : 4
}
]
}
[ EDIT - ADD ] : This is an answer after the question post's expected result is modified. This query uses the question post's sample documents as input.
db.collection.aggregate([
{
$group: {
_id: { libraryName: "$libraryName" },
count: { "$sum": 1 },
stages: { $push: "$stages" }
}
},
{
$unwind: "$stages"
},
{
$unwind: "$stages"
},
{
$group: {
_id: { libraryName: "$_id.libraryName", type: "$stages.type" },
type_count: { "$sum": 1 },
count: { $first: "$count" }
}
},
{
$group: {
_id: "$_id.libraryName",
count: { $first: "$count" },
stages: { $push: { type: "$_id.type", count: "$type_count" } }
}
},
])
The result:
{
"_id" : "a2",
"count" : 1,
"stages" : [
{
"type" : "b2",
"count" : 3
},
{
"type" : "b1",
"count" : 1
}
]
}
{
"_id" : "a1",
"count" : 2,
"stages" : [
{
"type" : "b2",
"count" : 4
},
{
"type" : "b3",
"count" : 1
},
{
"type" : "b1",
"count" : 3
}
]
}