MongoDB aggregate: count of elements in an object key - mongodb

Question

I have a MongoDB document like this:
[
{
"_id": {
"$oid": "5ff09030cd55d6d9f378d460"
},
"username": "a value",
"userid": "123456",
"last_access_ts": 1612426253,
"last_access": "2021-2-4 9:10:53",
"anotherid": 12345678910,
"verified_date": "2021-1-2 16:24:32",
"verified_ts": 1609601072,
"group_users": {
"-1001159747589": [
{
"anotherid": 12345678910,
"userid": "123456"
}
],
"-1001143137644": [
{
"anotherid": 12345678910,
"userid": "123456"
}
],
"-1001368608972": [
{
"anotherid": 12345678910,
"userid": "123456"
}
]
},
"registered_access": "2021-1-2 16:24:42",
}
]
I've two questions.
First one: I need to count the elements inside each group_users[key] object, and I'm stuck with this aggregate:
db.collection.aggregate([
{
$match: {
username: "a value"
}
},
{
$project: {
_id: 1,
userid: 1,
"groups": {
"$objectToArray": "$group_users"
}
}
},
{
$unwind: "$groups",
},
])
This aggregate gives me this result:
[
{
"_id": ObjectId("5ff09030cd55d6d9f378d460"),
"groups": {
"k": "-1001449720492",
"v": [
{
"anotherid": 12345678910,
"userid": "123456"
}
]
},
"userid": "123456"
},
{
"_id": ObjectId("5ff09030cd55d6d9f378d460"),
"groups": {
"k": "-1001159747589",
"v": [
{
"anotherid": 12345678910,
"userid": "123456"
}
]
},
"userid": "123456"
},
{
"_id": ObjectId("5ff09030cd55d6d9f378d460"),
"groups": {
"k": "-1001143137644",
"v": [
{
"anotherid": 12345678910,
"userid": "123456"
}
]
},
"userid": "123456"
}
]
How can I count each single groups[v] and then re-group the data? I would like to have a result like:
{
... some user data
"groups": {
"group_key": "count",
"second_group_key": "count",
"third_group_key": "count"
}
}
Is it possible with aggregate or I need to loop in the code?
My second question is always about the group_users. Is possible to have, recursively, the user data inside a group_users object?
I mean, every object inside group_users is an array of users; from this array can I have the user data (maybe with $graphLookup?) using the userid field or the anotherid field?
As a result from this second aggregate I would like to have something like this:
{
... some user data
"groups": {
"group_key": [{"userid": userid, "username": username}],
"second_group_key": [{"userid": userid, "username": username}],
"third_group_key": [{"userid": userid, "username": username}]
}
}
Obviously I can limit this "recursion" to 10 elements per time.
Thanks for any advice.

Answer 1

$objectToArray convert group_users object to array
$let to Bind variable group_arr for use in the specified expression, and returns the result of the expression.
$map to iterate loop of bind variable group_arr, get size of total element of v and return k and v,
$arrayToObject convert returned array from $map to object
db.collection.aggregate([
{ $match: { username: "a value" } },
{
$project: {
_id: 1,
userid: 1,
groups: {
$let: {
vars: { group_arr: { $objectToArray: "$group_users" } },
in: {
$arrayToObject: {
$map: {
input: "$$group_arr",
in: {
k: "$$this.k",
v: { $size: "$$this.v" }
}
}
}
}
}
}
}
}
])
Playground
Second question,
$unwind deconstruct groups array
$lookup with pipeline, match anotherid $in condition and return required fields
$group by _id and reconstruct groups array
$arrayToObject convert groups array to object
db.collection.aggregate([
{ $match: { username: "a value" } },
{
$project: {
_id: 1,
userid: 1,
groups: { $objectToArray: "$group_users" }
}
},
{ $unwind: "$groups" },
{
$lookup: {
from: "collection",
let: { anotherid: "$groups.v.anotherid" },
pipeline: [
{ $match: { $expr: { $in: ["$anotherid", "$$anotherid"] } } },
{
$project: {
_id: 0,
userid: 1,
username: 1
}
}
],
as: "groups.v"
}
},
{
$group: {
_id: "$_id",
groups: { $push: "$groups" },
userid: { $first: "$userid" }
}
},
{ $addFields: { groups: { $arrayToObject: "$groups" } } }
])
Playground