How to populate in 3 collection in mongoDB with Mongoose

Question

I have three collections such as User, Program, and `Agenda. Those models as follows.

User Model

const mongoose = require('mongoose');

const UserSchema = mongoose.Schema({
    name: {type:String},
    email: {type:String}
},{timestamps:true
}
);

module.exports = mongoose.model('User', UserSchema);

Program Model

const mongoose = require('mongoose');

const NoteSchema = mongoose.Schema({
    name: {type:String},
    timefrom: {type:Date},
    timeto: {type:Date},
    status: {type:String},
    venue: {type:String},
    timetype: {type:Number},
    userid:{type:mongoose.Schema.Types.ObjectId,ref : 'User', required: true},
    logo :{type:String,default: 'programe'}
},{timestamps:true
});

module.exports = mongoose.model('Program', NoteSchema);

Agenda Model

const mongoose = require('mongoose');

const AgendaSchema = mongoose.Schema({
    name: {type:String},
    timefrom: {type:Date},
    timeto: {type:Date},
    status: {type:String},
    proorder: {type:String},
    proid:{type:mongoose.Schema.Types.ObjectId,ref : 'Program', required: true}
},
{timestamps:true}
);

module.exports = mongoose.model('Agenda', AgendaSchema);

Now I only get agenda and program data only.

Agenda Controller

// Retrieve and return all agenda from the database.
exports.findAll = (req, res) => {

    Agenda.find()
    .populate('proid')
    //.populate('userid')

    .then(agendas => {
        res.send(agendas);
    }).catch(err => {
        res.status(500).send({
            message: err.message || "Some error occurred while retrieving agenda."
        });
    });
};

When I go to this URL and GET method I want to populate agenda document(done), related program document(done) and related user document(this I want)?

The wanted query as like this

SELECT * 
FROM users, programs, agendas
WHERE agendas.proid = programs.id AND programs.userid = users.id

Answer 1

You can either use $lookup aggregation

Agenda.aggregate([
  { "$lookup": {
    "from": Program.collection.name,
    "let": { "proid": "$proid" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$_id", "$$proid" ] } } },
      { "$lookup": {
        "from": User.collection.name,
        "let": { "userid": "$userid" },
        "pipeline": [
          { "$match": { "$expr": { "$eq": [ "$_id", "$$userid" ] } } },
        ],
        "as": "userid"
      }},
      { "$unwind": "$userid" }
    ],
    "as": "proid"
  }},
  { "$unwind": "$proid" }
])

Or with populate

Agenda.find()
  .populate([{ path: 'proid', populate: { path: 'userid' }}])

Both will give you the same result