MongoDB aggregate merging fields - mongodb

Question

I have a mongo Database I'll like to "join" two of them and then merge some other fields:
Let's see the schemas:
Students Schema (and data):
{
"_id": ObjectId("5fbd564981b1313de790b580"),
"name": "John Doe",
"age": "21",
"image": "https://XXXX/481.png",
"subjects": [
{
"_id": ObjectId("5fbd4e6881b1313de790b56b"),
"passed": true,
},
{
"_id": ObjectId("5fcb63fa8814d96876c687bf"),
}
],
"__v": NumberInt("1"),
}
and Subject schema:
{
"_id": ObjectId("5fbd4e6881b1313de790b56b"),
"course": 3,
"teacher": "John Smith",
"name": "Math",
},
{
"_id": ObjectId("5fcb63fa8814d96876c687bf"),
"name": "IT",
"course": 8,
"teacher": "John Peter",
}
What I'll like to make a query with the subjects (all info) of a student, also if the student have additional fields in subject like passed add it to the subject subdocument.
Here is my query till now:
db.students.aggregate([
{
$match:
{
_id : ObjectId('5fbd564981b1313de790b580')
}
},
{
$lookup :
{
from : "subjects",
localField : "subjects._id",
foreignField : "_id",
as : "FoundSubject"
}
}
]);
which correctly make the "join" but the merge is still missing, I got as result:
{
"_id": ObjectId("5fbd564981b1313de790b580"),
"name": "John Doe",
"age": "21",
"image": "https://XXXX/481.png",
"subjects": [
{
"_id": ObjectId("5fbd4e6881b1313de790b56b"),
"passed": true,
},
{
"_id": ObjectId("5fcb63fa8814d96876c687bf"),
}
],
"__v": NumberInt("1"),
"FoundSubject": [
{
"_id": ObjectId("5fbd4e6881b1313de790b56b"),
"course": 3,
"teacher": "John Smith",
"name": "Math"
},
{
"_id": ObjectId("5fcb63fa8814d96876c687bf"),
"name": "IT",
"course": 8,
"teacher": "John Peter"
}
]
}
but I'll like to have:
{
"_id": ObjectId("5fbd564981b1313de790b580"),
"name": "John Doe",
"age": "21",
"image": "https://XXXX/481.png",
"subjects": [
{
"_id": ObjectId("5fbd4e6881b1313de790b56b"),
"course": 3,
"teacher": "John Smith",
"name": "Math",
"passed": true,
},
{
"_id": ObjectId("5fcb63fa8814d96876c687bf"),
"name": "IT",
"course": 8,
"teacher": "John Peter"
}
],
"__v": NumberInt("1"),
}
with merged data and field "passed" added. How can accomplish that?
I'm new to MongoDB coming from MySQL.
Thanks

Answer 1

You need to merge both objects, add below stage after $lookup,
MongoDB Version From 3.4
$map to iterate loop of students array
$reduce to iterate loop of FoundSubject array, check condition if condition match then return required fields otherwise return initial value
$project to remove FoundSubject from result
{
$addFields: {
subjects: {
$map: {
input: "$subjects",
as: "s",
in: {
$reduce: {
input: "$FoundSubject",
initialValue: {},
in: {
$cond: [
{ $eq: ["$$s._id", "$$this._id"] },
{
_id: "$$this._id",
course: "$$this.course",
name: "$$this.name",
teacher: "$$this.teacher",
passed: "$$s.passed"
},
"$$value"
]
}
}
}
}
}
}
},
{ $project: { FoundSubject: 0 } }
Playground
MongoDB Version From 4.4
$map to iterate loop of students array,
$filter to get matching document from FoundSubject array and $first to get first object from array returned by filter
$mergeObjects to merge current objects with found result object from filter
remove FoundSubject using $$REMOVE
// skipping your stages
{
$addFields: {
FoundSubject: "$$REMOVE",
subjects: {
$map: {
input: "$subjects",
as: "s",
in: {
$mergeObjects: [
"$$s",
{
$first: {
$filter: {
input: "$FoundSubject",
cond: { $eq: ["$$s._id", "$$this._id"] }
}
}
}
]
}
}
}
}
}
Playground