CODEWITHSUNDEEP
mongodbaggregatelookup
Smith Dwayne
Smith Dwayne

Reputation: 2807

Mongodb aggregate lookup by _id is not working

MongoDb Aggregate lookup is not producing result while foreignField as _id. I have two collections say users and discussions

Sample users Data:

[{
    _id: 5f9c50dcfac1f091400225e3,
    email: '[email protected]',
    details: { fname: 'Peter Test', lname: 'Fulton' },
},
{
    _id: 5fa432bfb91fab7db60c70eb,
    email: '[email protected]',
    details: { fname: 'Frodo', lname: 'Baggins' },
},
{
    _id: 5fa8ec7d3ce22610e5d15190,
    email: '[email protected]',
    details: { fname: 'Tommy', lname: 'test' },
},
{
    _id: 5fc38bb0b3683651be970180,
    email: '[email protected]',
},
{
    _id: 5fd2340cc443d155ab38383b,
    email: '[email protected]',
    details: { fname: 'Dexter', lname: 'Lab' },
}]

Sample discussions data:

 {_id: ObjectId("5fb2abd6b14fa5683979df58"),
  tags: [ 'javascritp', 'css', 'html' ],
  title: 'Why is this inline-block element pushed downward?',
  post: 'Test Post',
  learnerId: ObjectId("5f9c50dcfac1f091400225e3"),
 }

Here '_id' of users is linked with 'learnerId' of 'discussions'.

My Aggregate query is like below.

db.users.aggregate([
            { $project: { "details.fname": 1, "details.lname":1,email:1, _id:1}},
            {$lookup: {
                from: "discussions",
                localField: "learnerId",
                foreignField: "_id",
                as: "discussions"
            }}
        ])

Here 'Peter Test' with _id 5f9c50dcfac1f091400225e3 linked with discussions LeanerId. But I expected discussions will populate in my result. My am seeing empty discussions array in all users collections.

 [{
    _id: 5f9c50dcfac1f091400225e3,
    email: '[email protected]',
    details: { fname: 'Peter Test', lname: 'Fulton' },
    discussions: []
},
{
    _id: 5fa432bfb91fab7db60c70eb,
    email: '[email protected]',
    details: { fname: 'Frodo', lname: 'Baggins' },
    discussions: []
},
{
    _id: 5fa8ec7d3ce22610e5d15190,
    email: '[email protected]',
    details: { fname: 'Tommy', lname: 'test' },
    discussions: []
},
{
    _id: 5fc38bb0b3683651be970180,
    email: '[email protected]',
    discussions: []
},
{
    _id: 5fd2340cc443d155ab38383b,
    email: '[email protected]',
    details: { fname: 'Dexter', lname: 'Lab' },
    discussions: []
}]

Can you point out what wrong in my aggregate query here?

Upvotes: 1

Views: 1290

Answers (1)

varman
varman

Reputation: 8894

You have mismatched the localField and foreignField

db.users.aggregate([
  {
    $project: {
      "details.fname": 1,
      "details.lname": 1,
      email: 1,
      _id: 1
    }
  },
  {
    $lookup: {
      from: "discussions",
      localField: "_id",
      foreignField: "learnerId",
      as: "discussions"
    }
  }
])

Working Mongo playground

Upvotes: 1

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