MongoDB aggregation query based on multiple fields with similar values - mongodb

Question

I have documents that look like this:
{
"_id" : "001",
"a" : {
"b" : {
"c" : {
"custId" : "cust1"
},
"d" : {
"custId" : "cust2"
}
}
}
}
{
"_id" : "002",
"a" : {
"b" : {
"c" : {
"custId" : "cust1"
},
"d" : {
"custId" : "cust3"
}
}
}
}
{
"_id" : "003",
"a" : {
"b" : {
"c" : {
"custId" : null
},
"d" : {
"custId" : "cust2"
}
}
}
}
{
"_id" : "004",
"a" : {
"b" : {
"c" : {
"custId" : null
},
"d" : {
"custId" : "cust1"
}
}
}
}
I would like to obtain an aggregation which shows a sorted count of customer ids, ignoring null customer ids, like this:
{
"_id" : "cust1",
"count" : 3,
"records" : [
"001", "002", "004"
]
}
{
"_id" : "cust2",
"count" : 2,
"records" : [
"001", "003"
]
}
{
"_id" : "cust3",
"count" : 1,
"records" : [
"002"
]
}
I think each document needs to be broken down into 1 or 2 customer based arrays than then unwound back into documents, but I have been unable to determine a workable solution.

Answer 1

make an array of custId, $map to iterate loop of b after converting from object to array using $objectToArray
$unwind deconstruct custIds array
$match to filter none null custIds documents
$group by custIds and get count of total records and make unique array of _id using $addToset
db.collection.aggregate([
{
$project: {
custIds: {
$map: {
input: { $objectToArray: "$a.b" },
in: "$$this.v.custId"
}
}
}
},
{ $unwind: "$custIds" },
{ $match: { custIds: { $ne: null } } },
{
$group: {
_id: "$custIds",
count: { $sum: 1 },
records: { $addToSet: "$_id" }
}
}
])
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