Data not getting fetched using aggregation query in nodejs

Question

I tried below code but only first match is checked and displayed,others displayed as object, why I am unable to see it in console. I have 3 collections student subject teacher, also made schema for the same. tried aggregation

Student.aggregate([

            {

              $match: { name: 'abcd'}
            },

            {
            $lookup:
             {
               from:'teachers',
               pipeline: [{ $match: { name: 'pqrs' } },],
               as: "teacherLookup"

             }
            },
            {
            $lookup:
             {
               from:'subjects',
               pipeline: [{ $match: { name: 'computer' } }],
               as: "subjectLookup"
             }
            }

          ]).then(function (res) {
            console.log(res); 
            res.forEach(function(students){
              let id = students._id;
              console.log(id+ ' got id ')

            }

    output
    student 
    name:'abcd' -- its fetched and other two not displaying values only shows object
    teacherLookup: [ [Object] ]
    subjectLookup: [ [Object] ]

Yes, because lookup generates the array output. You need to iterate over it to log. — Vijay Rajpurohit
– Vijay Rajpurohit, Commented Apr 27, 2020 at 17:02
how to specifically fetch only the _id from db with the name given. — learnNcode
– learnNcode, Commented Apr 27, 2020 at 20:19
{ $lookup: { from:'teachers', pipeline: [aggregation{{ $match: { name: 'pqrs' } },{_id:'1'}} ], as: "teacherLookup" } }, — learnNcode
– learnNcode, Commented Apr 27, 2020 at 20:21

Vijay Rajpurohit · Accepted Answer · 2020-04-28 05:29:47Z

1

You were there just to project something you have to use $project stage.

Here I'm adding the query:

Student.aggregate([
  {
    $match: { name: 'abcd'}
  },
  {
    $lookup:{
       from:'teachers',
       pipeline: [
        { 
          $match: { name: 'pqrs' } 
        },
        {
          $project:{
            "_id":1
          }
        }
       ],
       as: "teacherLookup"
    }
  },
  {
    $lookup:
     {
       from:'subjects',
       pipeline: [
        { 
          $match: { name: 'computer' } 
        },
        {
          $project:{
            "_id":1
          }
        }
       ],
       as: "subjectLookup"
     }
  }
])

For more about $project refer here.

Hope this will help :)

answered Apr 28, 2020 at 5:29

Vijay Rajpurohit

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