Alias for each "_id" in array - arrays

Question

How can i set alias for each "_id" in array of students.
I want to set an alias for _id in this json output.
{
"items": [
{
"id": "5d7aa7c1cba3435ebcb069c6",
"start": "2019-01-01T10:00:00.000Z",
"end": "2019-01-01T10:00:00.000Z",
"description": "test",
"students": [
{
"_id": "5d7aa779cba3435ebcb069c5", // <- alias _id to id
"name": "Jon",
"surname": "Snow"
}
]
}
]
}
How can i do that with Aggregation operations?

Answer 1

Use the native $map operator in your aggregate pipeline to transform the array. You would need to nest two $map operations; one
for the items array and an inner $map for the students array:
const studentsMap = {
'$map': {
'input': '$$this.students',
'as': 'student',
'in': {
'id': '$$student._id',
'name': '$$student.name',
'surname': '$$student.surname'
}
}
}
db.collection.aggregate([
{ '$addFields': {
'items': {
'$map': {
'input': '$items',
'in': {
'id': '$$this.id',
'start': '$$this.start',
'end': '$$this.end',
'description': '$$this.description',
'students': studentsMap
}
}
}
} }
])

Answer 2

Use map to transform.
items = items.map((item)=>{
item.students = item.students.map((student)=>{
student.id = student._id;
delete student._id;
return student;
})
return item;
})

Answer 3

A better approach would be to avoid hard coding of the fields in the $map stage as the maintenance of such queries becomes difficult when document size grows. The following query can get us the expected output and it's independent of the fields present in the document. As it would only focus on replacing the _id present in items.students.
db.collection.aggregate([
{
$addFields:{
"items":{
$map:{
"input":"$items",
"as":"item",
"in":{
$mergeObjects:[
"$$item",
{
"students":{
$map:{
"input":"$$item.students",
"as":"student",
"in":{
$mergeObjects:[
"$$student",
{
"id":"$$student._id"
}
]
}
}
}
}
]
}
}
}
}
},
{
$project:{
"items.students._id":0
}
}
]).pretty()