Mongo $lookup return empty array

Question

When performing a $lookup on my schemas, it always return an empty array. What am I doing wrong?

Result Collection

const resultSchema = new mongoose.Schema({
  trial: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Trial',
    required: true
  }
});

Trial Collection

const trialSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true
  }
});

Aggregate

Result.aggregate([
    {
      $lookup: {
        from: 'trial',
        localField: 'trial',
        foreignField: '_id',
        as: 'x'
      }
    }
  ])
    .exec()
    .then(results => ({ results }))

"x" ends up being always an empty array in the end.

0xkvn · Accepted Answer · 2018-03-18 15:03:12Z

3

Ok just found the answer right here: https://stackoverflow.com/a/45481516/3415561

The "from" field in lookup must be your collection name and not the model name. Therefore it's a plural word. Here it's

from: 'trials'

instead of

from: 'trial'

answered Mar 18, 2018 at 15:03

0xkvn

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1 Comment

Smit Patadiya Over a year ago

Also make sure that Field Type should be same. Example:- String with string, ObjectId with ObjectId

MalcolmOcean · Accepted Answer · 2019-02-07 14:52:22Z

1

In my case I had mistakenly put a $ in front of the collection name:

{$lookup: {
  from: '$users', // <- incorrect
  foreignField: '_id',
  localField: 'userid',
  as: 'users'
}},

it needed to be

{$lookup: {
  from: 'users', // <- correct
  foreignField: '_id',
  localField: 'userid',
  as: 'users'
}},

answered Feb 7, 2019 at 14:52

MalcolmOcean

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Comments

Artur P. · Accepted Answer · 2018-03-18 13:19:46Z

0

check collection name. In aggregate function 'trial' starts lowercase, in results collection 'Trial' starts uppercase

answered Mar 18, 2018 at 13:19

Artur P.

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3 Comments

0xkvn Over a year ago

Thanks for you answer! Changed it to 'Trial' in the from field of the lookup, doesn't have any effect. Plus, the populate function works on these schemas so I guess/hope it was not the problem.

Artur P. Over a year ago

Look at single document in Result Collection via mongodb console. Maybe mongoose use weird formating

0xkvn Over a year ago

Tried to delete the collections, corrected every uppercase characters from every models, recreated every documents. Same result. Also tried to reproduce query in mongo console, same result...

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