JAVASCRIPT

populate example in mongoose

const Live = new Schema(
  {
    user_id:{ 		// this one populate
            type: Schema.Types.ObjectId,      
            ref: 'User',    //  User Schema
            required: true                 
        },
        title: {
            type: String,
        },
        category:{        // this one populate
            type: Schema.Types.ObjectId,
            ref: 'Category'  // Category Schema
        },
  }
)
let vid = await Live.find({})
            .populate({path:'user_id',select:'name image -_id'})
            .populate({path:'category',select:'category -_id',model:Category})

what does populate in mongoose mean?

"Mongoose populate() lets you reference documents in other collections.
Population is the process of automatically replacing the specified 
paths in the document with document(s) from other collection(s)."

Source: https://mongoosejs.com/docs/populate.html

mongoose (populate)

// The 2nd `populate()` call below overwrites the first because they
// both populate 'fans'.
Story.
  find().
  populate({ path: 'fans', select: 'name' }).
  populate({ path: 'fans', select: 'email' });
// The above is equivalent to:
Story.find().populate({ path: 'fans', select: 'email' });

Mongoose populate example

const members = await subtable.find({ party: req.body.party }).populate({
    path: "user_id", //subtable field which refers to your main table
    select: "fname lname",
  });

mongoose + populate

app.get('/movies', passport.authenticate('jwt', { session: false }), (req, res) => {
	Movies.find()
		.populate('Genre')
		.populate('Actor')
		.populate('Director')
		.then((movies) => {
			res.status(201).json(movies);
		})
		.catch((err) => {
			console.error(err);
			res.status(500).send(`Error: ${err}`);
		});
});

mongoose populate()

// populates a single object
User.findById(id, function (err, user) {
  const opts = [
    { path: 'company', match: { x: 1 }, select: 'name' },
    { path: 'notes', options: { limit: 10 }, model: 'override' }
  ];

  User.populate(user, opts, function (err, user) {
    console.log(user);
  });
});

// populates an array of objects
User.find(match, function (err, users) {
  const opts = [{ path: 'company', match: { x: 1 }, select: 'name' }];

  const promise = User.populate(users, opts);
  promise.then(console.log).end();
})

// imagine a Weapon model exists with two saved documents:
//   { _id: 389, name: 'whip' }
//   { _id: 8921, name: 'boomerang' }
// and this schema:
// new Schema({
//   name: String,
//   weapon: { type: ObjectId, ref: 'Weapon' }
// });

const user = { name: 'Indiana Jones', weapon: 389 };
Weapon.populate(user, { path: 'weapon', model: 'Weapon' }, function (err, user) {
  console.log(user.weapon.name); // whip
})

// populate many plain objects
const users = [{ name: 'Indiana Jones', weapon: 389 }]
users.push({ name: 'Batman', weapon: 8921 })
Weapon.populate(users, { path: 'weapon' }, function (err, users) {
  users.forEach(function (user) {
    console.log('%s uses a %s', users.name, user.weapon.name)
    // Indiana Jones uses a whip
    // Batman uses a boomerang
  });
});
// Note that we didn't need to specify the Weapon model because
// it is in the schema's ref

populate in mongoose

const mongoose = require('mongoose');
const { Schema } = mongoose;

const personSchema = Schema({
  _id: Schema.Types.ObjectId,
  name: String,
  age: Number,
  stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});

const storySchema = Schema({
  author: { type: Schema.Types.ObjectId, ref: 'Person' },
  title: String,
  fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
});

const Story = mongoose.model('Story', storySchema);
const Person = mongoose.model('Person', personSchema);

how to use mongoose populate

{  _id: 59ab1c92ea84486fb4ba9f28,  username: JD,  posts: [    "59ab1b43ea84486fb4ba9ef0",    "59ab1b43ea84486fb4ba9ef1"  ]}