mongodb - Merge object arrays based on key - mongodb

Question

In a mongodb database, I have the following data:
// db.people
[
{
_id: ObjectId("..."),
id: 111111111,
name: "George",
relatedPeople: [{ id: 222222222, relation: "child" }],
// A bunch of other data I don't care about
},
{
_id: ObjectId("..."),
id: 222222222,
name: "Jacob",
relatedPeople: [{ id: 111111111, relation: "father" }],
// A bunch of other data I don't care about
},
{
_id: ObjectId("..."),
id: 333333333,
name: "some guy",
relatedPeople: [],
// A bunch of other data I don't care about
},
]
I would like to query the people, and select only the fields I've shown, but have extra data in relatedPeople (id + relation + name)
So the desired output would be:
[
{
_id: ObjectId("..."),
id: 111111111,
name: "George",
relatedPeople: [{ id: 222222222, relation: "child", name: "Jacob" }],
},
{
_id: ObjectId("..."),
id: 222222222,
name: "Jacob",
relatedPeople: [{ id: 111111111, relation: "father", name: "George" }],
},
{
_id: ObjectId("..."),
id: 333333333,
name: "some guy",
relatedPeople: [],
},
]
I can get something close, with this query:
db.people.aggregate([
// { $match: { /** ... */ }, },
{
$lookup: {
from: "people",
let: { relatedPeopleIds: "$relatedPeople.id" },
pipeline: [
{ $match: { $expr: { $in: ["$id", "$$relatedPeopleIds"] } } },
{
$project: {
id: 1,
name: 1,
},
},
],
as: "relatedPeople2",
},
},
{
$project: {
id: 1,
name: 1,
relatedPeople: 1,
relatedPeople2: 1,
}
}
]);
But the data is split between two fields. I want to merge each object in the arrays by their id, and place the result array in relatedPeople
I found this question, but that merge is done over a range and uses $arrayElementAt which I can't use
I also tried looking at this question, but I couldn't get the answer to work (Kept getting empty results)

Answer 1

You can add one step using $arrayElementAt with $indexOfArray:
db.people.aggregate([
// { $match: { /** ... */ }, },
{$project: {id: 1, name: 1, relatedPeople: 1}},
{$lookup: {
from: "people",
let: { relatedPeopleIds: "$relatedPeople.id" },
pipeline: [
{ $match: { $expr: { $in: ["$id", "$$relatedPeopleIds"] } } },
{
$project: {
id: 1,
name: 1,
},
},
],
as: "relatedPeople2",
},
},
{$set: {
relatedPeople: {$map: {
input: "$relatedPeople",
in: {$mergeObjects: [
"$$this",
{$arrayElemAt: [
"$relatedPeople2",
{$indexOfArray: ["$relatedPeople2.id", "$$this.id"]}
]}
]}
}}
}},
{$unset: "relatedPeople2"}
])
See how it works on the playground example