$lookup when foreignField is in nested array

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

score 4 · Accepted Answer · answered Jun 18 '18 at 16:21

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'
    }
}])

hey thanks, @dnickless I understand what I was missing in the query, Thanks a lot — Arjun Shrivastava, Jun 19 '18 at 06:34
Your comment about "$expr cannot digest arrays..." really helped me. — pberggreen, Feb 27 '20 at 15:05
could you pls help on this: https://stackoverflow.com/questions/61109179/mongodb-find-deeply-nested-object-with-lookup?noredirect=1#comment108111460_61109179 — newdeveloper, Apr 08 '20 at 22:52

Ashh · Answer 2 · 2018-06-19T13:02:24.830

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"
  }}
])

score 1 · Answer 3 · answered Jun 18 '18 at 14:58

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>, ... }
  ] 
}

$lookup when foreignField is in nested array

3 Answers3

Linked