Combine Mongo Documents after multiple lookups in single aggregation - arrays

Question

I'm stuck trying to combine my document results. Here is my query and data
{"_id":"5c21ab13d03013b384f0de26",
"roles":["5c21ab31d497a61195ce224c","5c21ab4ad497a6f348ce224d","5c21ab5cd497a644b6ce224e"],
"agency":"5b4ab7afd6ca361cb38d6a60","agents":["5b4ab5e897b24f1c4c8e3de3"]}
Here is the query
return db.collection('projects').aggregate([
{
$match: {
agents: ObjectId(agent)
}
},
{
$unwind: "$agents"
},
{
$lookup: {
from: "agents",
localField: "agents",
foreignField: "_id",
as: "agents"
}
},
{
$unwind: {
path: "$roles",
preserveNullAndEmptyArrays: true
}
},
{
$lookup: {
from: "roles",
localField: "roles",
foreignField: "_id",
as: "roles"
}
},
{
$lookup: {
from: "agencies",
localField: "agency",
foreignField: "_id",
as: "agency"
}
}
])
As you can see, an entry in the project collection has two arrays that are unwound before a lookup on each entry is performed and then a final lookup is performed on the "agency" field.
However when I get the results from this query I am getting a document count equal to the number of roles. For example the project I am aggregating has 3 roles and 1 agent. So I am getting back an array of 3 objects, one for each role rather than a single document with the roles array containing all three roles. There is also a chance the agents array can have more than one value.
So lost...

Answer 1

You don't have to run $unwind before $lookup. The localField section states that:
If your localField is an array, you may want to add an $unwind stage to your pipeline. Otherwise, the equality condition between the localField and foreignField is foreignField: { $in: [ localField.elem1, localField.elem2, ... ] }
So basically if you don't run $unwind for instance on roles then instead of document per role you will get an array of roles as ObjectIds replaced by an array of objects from that second collection.
So you can try following aggregation:
db.collection('projects').aggregate([
{
$match: {
agents: ObjectId(agent)
}
},
{
$lookup: {
from: "agents",
localField: "agents",
foreignField: "_id",
as: "agents"
}
},
{
$lookup: {
from: "roles",
localField: "roles",
foreignField: "_id",
as: "roles"
}
},
{
$lookup: {
from: "agencies",
localField: "agency",
foreignField: "_id",
as: "agency"
}
}
])