MongoDB Aggregation With $lookup for field inside an array - mongodb

Question

Ive been trying to make a MongoDB query with aggregation to produce a data in a structure I like. So i have to collections namely People and Calendar
People
_id : ObjectId,
name: string,
age: number,
occupation: string
address
Calendar
_id: ObjectId,
months: [
{
month: Date,
users: [
{
PersonId: ObjectId(Reference from People._id)
...some other details
},
{
PersonId: ObjectId(Reference from People._id)
...some other details
},
{
PersonId: ObjectId(Reference from People._id)
...some other details
},
]
},
{
month: Date,
users: [same as above]
},
{
month: Date,
users: [same as above]
},
]
So, what I would like the final data to look is like below.
Final Result
Calendar
_id: ObjectId,
months: [
{
month: Date,
users: [
{
PersonId: ObjectId(Reference from People._id)
Person : data from Person Collection
...some other details
},
{
PersonId: ObjectId(Reference from People._id)
Person : data from Person Collection
...some other details
},
{
PersonId: ObjectId(Reference from People._id)
Person : data from Person Collection
...some other details
},
]
},
{
month: Date,
users: [same as above]
},
{
month: Date,
users: [same as above]
},
]
I have been trying with below code but could not get it right.
.aggregate([
{$unwind: "$months"},
{$unwind: "$months.users"},
{
$lookup:
{
from: "Person",
localField: "months.users.PersonId",
foreignField: "_id",
as: "PersonList"
}
},
{$unwind: "$PersonList"},
{
$group:
{
"_id": "$_id",
}
}
])
How do I achieve this?
Is this something do-able?

Answer 1

So you can do something like this,
db.calender.aggregate([
{
$unwind: "$months"
},
{
$unwind: "$months.users"
},
{
$lookup: {
from: "people",
localField: "months.users.personId",
foreignField: "_id",
as: "months.users.people"
}
},
{
$group: {
"_id": {
_id: "$_id",
months: "$months.month"
},
users: {
$addToSet: "$months.users"
}
}
},
{
$group: {
_id: "$_id._id",
months: {
$push: {
month: "$_id.months",
users: "$users"
}
}
}
}
])
You need to add the details to nested array, So we use unwind two times, when we re-group it, we need to use two groups. The months array is uniquely identify by a unique field. (As you said months.month is unique, if it is not unique, give another filed _id which should be unique)
Working Mongo playground