$lookup when foreignField is in nested array - mongodb

Question

I have two collections :
Student
{
_id: ObjectId("657..."),
name:'abc'
},
{
_id: ObjectId("593..."),
name:'xyz'
}
Library
{
_id: ObjectId("987..."),
book_name:'book1',
issued_to: [
{
student: ObjectId("657...")
},
{
student: ObjectId("658...")
}
]
},
{
_id: ObjectId("898..."),
book_name:'book2',
issued_to: [
{
student: ObjectId("593...")
},
{
student: ObjectId("594...")
}
]
}
I want to make a Join to Student collection that exists in issued_to array of object field in Library collection.
I would like to make a query to student collection to get the student data as well as in library collection, that will check in issued_to array if the student exists or not if exists then get the library document otherwise not.
I have tried $lookup of mongo 3.6 but I didn`t succeed.
db.student.aggregate([{$match:{_id: ObjectId("593...")}}, $lookup: {from: 'library', let: {stu_id:'$_id'}, pipeline:[$match:{$expr: {$and:[{"$hotlist.clientEngagement": "$$stu_id"]}}]}])
But it thorws error please help me in regard of this. I also looked at other questions asked at stackoverflow like. question on stackoverflow,
question2 on stackoverflow but these are comapring simple fields not array of objects. please help me

Answer 1

I am not sure I understand your question entirely but this should help you:
db.student.aggregate([{
$match: { _id: ObjectId("657...") }
}, {
$lookup: {
from: 'library',
localField: '_id' ,
foreignField: 'issued_to.student',
as: 'result'
}
}])
If you want to only get the all book_names for each student you can do this:
db.student.aggregate([{
$match: { _id: ObjectId("657657657657657657657657") }
}, {
$lookup: {
from: 'library',
let: { 'stu_id': '$_id' },
pipeline: [{
$unwind: '$issued_to' // $expr cannot digest arrays so we need to unwind which hurts performance...
}, {
$match: { $expr: { $eq: [ '$issued_to.student', '$$stu_id' ] } }
}, {
$project: { _id: 0, "book_name": 1 } // only include the book_name field
}],
as: 'result'
}
}])

Answer 2

This might not be a very good answer, but if you can change your schema of Library to:
{
_id: ObjectId("987..."),
book_name:'book1'
issued_to: [
ObjectId("657..."),
ObjectId("658...")
]
},
{
_id: "ObjectId("898...")",
book_name:'book2'
issued_to: [
ObjectId("593...")
ObjectId("594...")
]
}
Then when you do:
{
$lookup: {
from: 'student',
localField: 'issued_to',
foreignField: '_id',
as: 'issued_to_students', // this creates a new field without overwriting your original 'issued_to'
}
},
You should get, based on your example above:
{
_id: ObjectId("987..."),
book_name:'book1'
issued_to_students: [
{ _id: ObjectId("657..."), name: 'abc', ... },
{ _id: ObjectId("658..."), name: <name of this _id>, ... }
]
},
{
_id: "ObjectId("898...")",
book_name:'book2'
issued_to: [
{ _id: ObjectId("593..."), name: 'xyz', ... },
{ _id: ObjectId("594..."), name: <name of this _id>, ... }
]
}

Answer 3

You need to $unwind the issued_to from library collection to match the issued_to.student with _id
db.student.aggregate([
{ "$match": { "_id": mongoose.Types.ObjectId(id) } },
{ "$lookup": {
"from": Library.collection.name,
"let": { "studentId": "$_id" },
"pipeline": [
{ "$unwind": "$issued_to" },
{ "$match": { "$expr": { "$eq": [ "$issued_to.student", "$$studentId" ] } } }
],
"as": "issued_to"
}}
])