Mongodb - Aggregation over multiple collections specific result format - mongodb

Question

I need to create an aggregation using information from multiple collections and arrange them in a specific result format. Here we have three collections (parent, task, slots; the model of these cannot be changed anymore). Each parent can have many tasks, which consist of different slots (see the simplified sample data below)
Slot collection
[
{
_id: ObjectId('6015720974cff84db3499cad'),
name: 'Test 1',
start: ISO-Date('2021-07-13T09:00:00.000+00:00'),
end: ISO-Date('2021-07-13T09:15:00.000+00:00'),
parentId: 1,
taskId: 1
},
{
_id: ObjectId('6015720974cff84db3478cad'),
name: 'Test 2',
start: ISO-Date('2021-07-13T09:15:00.000+00:00'),
end: ISO-Date('2021-07-13T09::30.000+00:00'),
parentId: 1,
taskId: 1
},
{
_id: ObjectId('6015720974cff84db3499bcd'),
name: 'Test 3',
start: ISO-Date('2021-07-13T11:00:00.000+00:00'),
end: ISO-Date('2021-07-13T11:15:00.000+00:00'),
parentId: 1,
taskId: 2
},
{
_id: ObjectId('6015720974cff84db3499efg'),
name: 'Test 4',
start: ISO-Date('2021-07-13T13:00:00.000+00:00'),
end: ISO-Date('2021-07-13T13:15:00.000+00:00'),
parentId: 2,
taskId: 3
},
{
_id: ObjectId('6015720974cff84db349967e'),
name: 'Test 5',
start: ISO-Date('2021-07-13T13:15:00.000+00:00'),
end: ISO-Date('2021-07-13T13:30:00.000+00:00'),
parentId: 2,
taskId: 3
},
]
Task collection
[
{
_id: ObjectId('6015720974cff84db3499c87'),
name: 'Task 1',
taskId: 1,
},
{
_id: ObjectId('6015720974cff84db3499b6b'),
name: 'Task 2',
taskId: 2,
},
{
_id: ObjectId('6015720974cff84db3499b19'),
name: 'Task 3',
taskId: 3,
}
]
Parent collection
[
{
_id: ObjectId('6015720974cff84db34995a6'),
name: 'Parent 1',
parentId: 1
},
{
_id: ObjectId('6015720974cff84db349962f'),
name: 'Parent 2',
parentId: 2
}
]
The result should have the following format:
[
{
parentName: 'Parent 1',
tasks: [
{
taskName: 'Task 1',
slots: [
{
name: 'Test 1',
start: ISO-Date('2021-07-13T09:00:00.000+00:00'),
end: ISO-Date('2021-07-13T09:15:00.000+00:00'),
},
{
name: 'Test 2',
start: ISO-Date('2021-07-13T09:15:00.000+00:00'),
end: ISO-Date('2021-07-13T09::30.000+00:00'),
},
]
},
{
taskName: 'Task 2',
slots: [
{
name: 'Test 3',
start: ISO-Date('2021-07-13T11:00:00.000+00:00'),
end: ISO-Date('2021-07-13T11:15:00.000+00:00'),
}
]
}
]
},
{
parentName: 'Parent 2',
tasks: [
{
taskName: 'Task 3',
slots: [
{
name: 'Test 4',
start: ISO-Date('2021-07-13T13:00:00.000+00:00'),
end: ISO-Date('2021-07-13T13:15:00.000+00:00'),
},
{
name: 'Test 5',
start: ISO-Date('2021-07-13T13:15:00.000+00:00'),
end: ISO-Date('2021-07-13T13:30:00.000+00:00'),
}
]
}
]
}
]
How can I achieve this result structure by given data? The lookup part is not a big deal, but I currently struggle to achieve this structure, as I'm very new to MongoDB aggregation framework. Thank you very much for any hints.

Answer 1

You can use lookup to join collections
$lookup to join collections
$unwind to deconstruct the array
$group to reconstruct the array
Here is the code,
db.Task.aggregate([
{
"$lookup": {
"from": "Slot",
"localField": "taskId",
"foreignField": "taskId",
"as": "slots"
}
},
{ "$unwind": "$slots" },
{
"$lookup": {
"from": "Parent",
"localField": "slots.parentId",
"foreignField": "parentId",
"as": "parents"
}
},
{ "$unwind": "$parents" },
{
$group: {
_id: { pId: "$parents._id", tId: "$taskId" },
parentName: { $first: "$parents.name" },
taskName: { "$first": "$name" },
slots: { $push: "$slots" }
}
},
{
$group: {
_id: "$_id.pId",
parentName: { $first: "$parentName" },
tasks: {
$push: {
taskName: "$taskName",
slots: "$slots"
}
}
}
}
])
Working Mongo playground