MongoDB: Assign document objects to field in '$project' stage - mongodb

Question

I have a user collection:
[
{"_id": 1,"name": "John", "age": 25, "valid_user": true}
{"_id": 2, "name": "Bob", "age": 40, "valid_user": false}
{"_id": 3, "name": "Jacob","age": 27,"valid_user": null}
{"_id": 4, "name": "Amelia","age": 29,"valid_user": true}
]
I run a '$facet' stage on this collection. Checkout this MongoPlayground.
I want to talk about the first output from the facet stage. The following is the response currently:
{
"user_by_valid_status": [
{
"_id": false,
"count": 1
},
{
"_id": true,
"count": 2
},
{
"_id": null,
"count": 1
}
]
}
However, I want to restructure the output in this way:
"analytics": {
"invalid_user": {
"_id": false
"count": 1
},
"valid_user": {
"_id": true
"count": 2
},
"user_with_unknown_status": {
"_id": null
"count": 1
}
}
The problem with using a '$project' stage along with 'arrayElemAt' is that the order may not be definite for me to associate an index with an attribute like 'valid_users' or others. Also, it gets further complicated because unlike the sample documents that I have shared, my collection may not always contain all the three categories of users.
Is there some way I can do this?

Answer 1

You can use $switch conditional operator,
$project to show value part in v with _id and count field as object, k to put $switch condition
db.collection.aggregate([
{
"$facet": {
"user_by_valid_status": [
{
"$group": {
"_id": "$valid_user",
"count": { "$sum": 1 }
}
},
{
$project: {
_id: 0,
v: { _id: "$_id", count: "$count" },
k: {
$switch: {
branches: [
{ case: { $eq: ["$_id", null] }, then: "user_with_unknown_status" },
{ case: { $eq: ["$_id", false] }, then: "invalid_user" },
{ case: { $eq: ["$_id", true] }, then: "valid_user" }
]
}
}
}
}
],
"users_above_30": [{ "$match": { "age": { "$gt": 30 } } }]
}
},
$project stage in root, convert user_by_valid_status array to object using $arrayToObject
{
$project: {
analytics: { $arrayToObject: "$user_by_valid_status" },
users_above_30: 1
}
}
])
Playground