Aggregate $lookup Array of Objects - mongodb

Question

I have collection schools with field groups and I am attempting to $lookup documents from the users collection. I am getting empty results however and an extra schools document.
Schools Schema
const SchoolSchema = new Schema({
groups: [
{
name: { type: String },
color: { type: String },
userDrivenName: { type: String },
},
]
});
module.exports = School = mongoose.model("School", SchoolSchema);
User Schema
const UserSchema = new Schema({
name: {
type: String,
required: true,
},
groups: [
{
groupId: { type: String },
name: { type: String },
color: { type: String },
userDrivenName: { type: String },
},
]
});
Query
db.schools.aggregate([
{
$match: {
_id: ObjectId("5d836e584a24e20e6090fd7b")
}
},
{
$project: {
groups: 1
}
},
{
$unwind: "$groups"
},
{
$lookup: {
from: "users",
let: {
groupId: "$groups._id"
},
pipeline: [
{
$match: {
"groups.groupId": "$$groupId"
}
}
],
as: "groups",
},
},
])
Results:
[
{
"_id": "5d836e584a24e20e6090fd7b",
"groups": []
},
{
"_id": "5d836e584a24e20e6090fd7b",
"groups": []
}
]
Expected Results:
[
{
"_id":"5d836e584a24e20e6090fd7b",
"groups":[
{
"_id":"5ec01fdc1dfb0a4f08316dfe",
"name":"GROUP 1",
"users":[
{
"name":"Luke Skywalker"
}
]
}
]
}
]
MongoPlayground

Answer 1

Two things:
There's a type mismatch between groupId and groups.groupId so you need to use $toString (based on your Mongo Playground example),
$lookup with custom pipelines allows only expression when you use $match so you need $in and $expr:
{
$lookup: {
from: "users",
let: { groupId: { $toString: "$groups._id" } },
pipeline: [
{
$match: {
$expr: {
$in: ["$$groupId","$groups.groupId"]
}
}
}
],
as: "groups"
}
}
Mongo Playground