How to add ObjectId argument to a mongodb lookup - mongodb

Question

I am doing a lookup where the local field is an ObjectId and the foreign field is an array of ObjectId's. Performing the lookup gives me the error:
arguments to $lookup must be strings
I have done similar lookups where the foreign field is not an array (but is an ObjectId) so the error seems to be ambiguous. my database consists two collections: Song and Playlist. A Song can belong to many playlists. I am trying to write an aggregation that returns a matching song that contains an array of playlists the song belongs to:
Songs:
[
{
songName: "In Da Club",
_id: ObjectId(1)
},
{
songName: "Happy Birthday",
_id: ObjectId(2)
},
{
songName: "Ode to Joy",
_id: ObjectId(3)
}
]
Playlists:
[
{
_id: ObjectId(4)
playlistName: "PlaylistOne,
songs: [ObjectId(1), ObjectId(3)]
},
{
_id: ObjectId(5)
playlistName: "PlaylistTwo,
songs: [ObjectId(1)]
}
]
Desired outcome:
{
songName: "In Da Club",
_id: ObjectId(1),
playlists: [
{
_id: ObjectId(4),
playlistName: "PlaylistOne,
},
{
_id: ObjectId(5),
playlistName: "PlaylistTwo"
}
]
}
The query I tried:
db.songs.aggregate([
{
$match: {
songName: "In Da Club"
}
},
{
$lookup: {
from: 'playlists',
let: { songId: '$_id'},
pipeline: [
{
$match: {
$expr: {
{
$in: ["$$songId", "$songs"]
}
}
}
}
],
as: 'playlists'
}
}
])
It seems to be a relatively simple query and I'm not sure how I can get around the "arguments passed into lookup must be strings" error since my lookup is based on ObjectId's. Any help would be greatly appreciated! TIA!

Answer 1

Though $lookup is introduced in version 3.2 but couple of enhancements are done on it in sub-sequent updates :
As per docs :
Starting MongoDB 3.4, if the localField is an array, you can match the
array elements against a scalar foreignField without needing an
$unwind stage.
MongoDB 3.6, adds support for executing a pipeline on the joined
collection, which allows for specifying multiple join conditions as
well as uncorrelated sub-queries.
So the issue might be from mongodb version being low. Anyway for your requirement you can use $lookup which is used for single equality join as array is on foreignField.
{
$lookup: {
from: "playlists",
localField: "_id", // Scalar value
foreignField: "songs", // Against an array
as: "playlists"
}
}
Test : mongoplayground