How to do mongodb inner join and grouping - mongodb

Question

// orders
[
{
"id": 1,
"orderName": "a",
"seqId": 100,
"etc": [],
"desc": [],
},
{
"id": 2,
"orderName": "b",
"seqId": 200,
"etc": [],
"desc": []
},
{
"id": 3,
"orderName": "c",
"seqId": 100,
},
]
// goods collection
[
{
"id": 1,
"title": "example1",
"items": [
{
"id": 10,
"details": [
{
"id": 100
},
{
"id": 101,
}
]
},
{
"id": 20,
"details": [
{
"id": 102,
},
{
"id": 103,
}
]
},
]
},
[
{
"id": 2,
"title": "example2",
"items": [
{
"id": 30,
"details": [
{
"id": 200
},
{
"id": 201
}
]
},
{
"id": 40,
"details": [
{
"id": 202
},
{
"id": 203
}
]
},
]
},
]
When the etc field and desc field arrays of the orders collection are empty, or the non-empty document's seqId field value and the goods collection's "goods.details.id field value are the same.
I want to express the sum operation based on the title of the product collection and the sum if it is not empty.
{example1: 1, total: 2}
{example2: 1, total: 1}
For example, "example1" and "example2" represent the sum of the cases where the etc and desc field arrays are empty (the title of the goods collection), and the total represents the total regardless of whether the array is empty or not.
If so, it should be marked aboveas:

Answer 1

Following our discussion here, we can remove the early filtering for the 2 empty arrays and move it to a conditional sum at the $group stage.
db.orders.aggregate([
{
"$lookup": {
"from": "goods",
"localField": "seqId",
"foreignField": "items.details.id",
"as": "goodsLookup"
}
},
{
"$unwind": "$goodsLookup"
},
{
$group: {
_id: "$goodsLookup.title",
emptySum: {
$sum: {
"$cond": {
"if": {
$and: [
{
$eq: [
"$desc",
[]
]
},
{
$eq: [
"$etc",
[]
]
}
]
},
"then": 1,
"else": 0
}
}
},
total: {
$sum: 1
}
}
}
])
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