Populate Virtuals does not seem to work. Could anyone show me the error?

Question

I am studying the Populate Virtuals: https://mongoosejs.com/docs/populate.html#populate-virtuals

require("./connection");

// //----------------------------------------------------
const mongoose = require("mongoose");
const Schema = mongoose.Schema;

const PersonSchema = new Schema({
  name: String,
  band: String
});

const BandSchema = new Schema({
  name: String
});

BandSchema.virtual("members", {
  ref: "Person", // The model to use
  localField: "name", // Find people where `localField`
  foreignField: "band", // is equal to `foreignField`
  // If `justOne` is true, 'members' will be a single doc as opposed to
  // an array. `justOne` is false by default.
  justOne: false,
  options: { sort: { name: -1 }, limit: 5 } 
});

const Person = mongoose.model("Person", PersonSchema);
const Band = mongoose.model("Band", BandSchema);

/**
 * Suppose you have 2 bands: "Guns N' Roses" and "Motley Crue"
 * And 4 people: "Axl Rose" and "Slash" with "Guns N' Roses", and
 * "Vince Neil" and "Nikki Sixx" with "Motley Crue"
 */
// Person.create([
//   {
//     name: "Axl Rose",
//     band: "Guns N' Roses"
//   },
//   {
//     name: "Slash",
//     band: "Guns N' Roses"
//   },
//   {
//     name: "Vince Neil",
//     band: "Motley Crue"
//   },
//   {
//     name: "Nikki Sixx",
//     band: "Motley Crue"
//   }
// ]);

// Band.create([{ name: "Motley Crue" }, { name: "Guns N' Roses" }]);
/////////////////////////////////////////////////////////////////////////

Band.find({})
  .populate("members")
  .exec(function(error, bands) {
    /* `bands.members` is now an array of instances of `Person` */
    console.log(bands.members);
  });

However, the output of this code is undefined; the mongoose tutorial claims it should be "an array of instances of Person".

I have tested another code, but I have gotten similar results: http://thecodebarbarian.com/mongoose-virtual-populate.html

First: could anyone let me know what is wrong on this code? I cannot see it!

Second: if it is not asking too much, could anyone try to show me the importance of this technique. They claim it is better than conventional populate regarding speed, I cannot see it at my current mongoose knowledge!

Related question: Mongoosejs virtual populate

Answer 1

The answer was right in front of my eyes! The question I have related to, namely, has the answer after all: Mongoosejs virtual populate

In order to keep the learning spirit, I shall summarize here the solution.

According to the official documentation:

If you use toJSON() or toObject() mongoose will not include virtuals by default.

See: https://mongoosejs.com/docs/guide.html#virtuals.

Honestly, I have no idea what that means, but it was included in a comment, and seems the answer!

So, what ones has to do is include the line:

BandSchema.virtual("members", {
  ref: "Person", // The model to use
  localField: "name", // Find people where `localField`
  foreignField: "band", // is equal to `foreignField`
  // If `justOne` is true, 'members' will be a single doc as opposed to
  // an array. `justOne` is false by default.
  justOne: false,
  options: { sort: { name: -1 }, limit: 5 }
});
//----------------------------------------------------------------    
//here, this one! 
    BandSchema.set("toObject", { virtuals: true });
    BandSchema.set("toJSON", { virtuals: true });

//------------------------------------------

I have been studying the mongoose official tutorial, "seriously?! guys, you can improve!" I have found it challenging.