MongoDB $lookup on array of documents containing object id - mongodb

Question

I want to make aggregation pipeline in MongoDB v4.4.
I have two collections.
users collection
[
{ _id: "user1", username: "john" },
{ _id: "user2", username: "bob" },
]
articles collection
[
{
_id: "article1",
view_history: [
{ user: "user1", viewed_at: ISODate("...") },
{ user: "user2", viewed_at: ISODate("...") },
{ user: "user1", viewed_at: ISODate("...") },
]
}
]
My desired result after running aggregation on articles collection is:
[
{
_id: "article1",
view_history: [
{ user: "john", viewed_at: ISODate("...") },
{ user: "bob", viewed_at: ISODate("...") },
{ user: "john", viewed_at: ISODate("...") },
]
}
]
So user fields on each history item in view_history are substituted with the user's username.
I tried many answers on SO, but can't find any solution for my situation.
Here's what I got so far:
db.articles.aggregate([
{
$lookup: {
from: "users",
localField: "view_history.user",
foreignField: "_id",
as: "output",
}
}
])
Which yields:
[
{
_id: "article1",
view_history: [
{ user: "user1", viewed_at: ISODate("...") },
{ user: "user2", viewed_at: ISODate("...") },
{ user: "user1", viewed_at: ISODate("...") },
],
output: [
{ _id: "user1", username: "john" },
{ _id: "user2", username: "bob" },
]
}
]
(There's only one user1 in output, and viewed_at not included)

Answer 1

$unwind deconstruct view_history array
$lookup with users collection
get first element from view_history.user.username because it will return array using $arrayElemAt
$group by _id and re-construct view_history array
db.articles.aggregate([
{ $unwind: "$view_history" },
{
$lookup: {
from: "users",
localField: "view_history.user",
foreignField: "_id",
as: "view_history.user"
}
},
{
$addFields: {
"view_history.user": {
$arrayElemAt: ["$view_history.user.username", 0]
}
}
},
{
$group: {
_id: "$_id",
view_history: {
$push: "$view_history"
}
}
}
])
Playground
Second approach:
$lookup with users collection
$map to iterate loop of view_history array
$filter to iterate loop of output array and match user field
$let declare variable u for above filter, in to get first element from filtered result using $arrayElemAt
db.articles.aggregate([
{
$lookup: {
from: "users",
localField: "view_history.user",
foreignField: "_id",
as: "output"
}
},
{
$project: {
view_history: {
$map: {
input: "$view_history",
as: "v",
in: {
user: {
$let: {
vars: {
u: {
$filter: {
input: "$output",
cond: { $eq: ["$$this._id", "$$v.user"] }
}
}
},
in: { $arrayElemAt: ["$$u.username", 0] }
}
},
viewed_at: "$$v.viewed_at"
}
}
}
}
}
])
Playground