Mongoose equivalent of JOIN … WHERE

Question

I've got two models

Opinion {
  _id: string;
  creator: string;
  teacher: ObjectId;
  text: string;
  date: Date;
}

Teacher {
  _id: string;
  name: string;
  isVerified: boolean;
}

I need to get 3 latest Opinions WHERE { teacher.isVerified: true }

I tried

    const newOpinions = await Opinion.find(
      null,
      "creator teacher text",
      {
        sort: {
          date: -1,
        },
      }).populate(
        {
          path: 'teacher',
          model: 'Teacher',
          select: 'name',
          match: {
            isVerified: true,
          },
        }
      ).limit(3);

but (after short analysis) it works as designed - I'm getting 3 latest opinions, no matter if teachers is verified or not (in teacher field I'm getting null if it's not verified)

Can anyone try to point me in the right direction? I think maybe something with Model.aggregate() .

Answer 1

This is one way to do it. With this one you will filter the teachers first. Please also consult $lookup documentation

Teacher.aggregate([{
  $match: { isVerified: true }
}, {
  $lookup: {
    from: 'opinions' // assume you have collection named `opinions` for model `Opinion`,
    localField: '_id', // join teacher._id ...
    foreignField: 'teacher', // ... with opionion.teacher
    as: 'opinions'
  } // from here you well get verified teachers with embedded opinions, 
// further stages are optional. We will modify the shape of output to be opinions only
}, { 
  $unwind: '$opinions' // spread out opinions array into separate documents
}, {
  $replaceRoot: '$opinions' // replace the root document with opinion only
}])

Answer 2

The equivalent of join in mongodb is $lookup. A mongoose way is to use populate but you'll have to provide ref key in your model

For lookup usage https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/

Mongoose Ref

Opinion {
  _id: string;
  creator: string;
  teacher: {
      type: Schema.Types.ObjectId, ref: 'Teacher'
  },
  text: string;
  date: Date;
}

Teacher {
  _id: string;
  name: string;
  isVerified: boolean;
}

The $lookup approach is much more flexible and customizable