relationships.js

const Sequelize = require('sequelize');
const db = new Sequelize('postgres://localhost:5432/my-grace-shopper');

// Barebones model definitions ...

const User = db.define('user', {
    email: Sequelize.STRING
});

const Album = db.define('album', {
    title: Sequelize.STRING
});

const Song = db.define('song', {
    title: Sequelize.STRING
});

const Artist = db.define('artist', {
    name: Sequelize.STRING
});

// End barebones model definitions ----

Song.belongsTo(Album);
// Places albumId column on song rows
// Allows song.getAlbum/setAlbum/removeAlbum to exist and function

Album.hasMany(Song);
// Also places albumId column on song rows, which is redundant to Song.belongsTo(Album)
// However, it also allows for the use of album.getSongs/album.setSong(s)/addSong(s)/etc

Artist.hasMany(Album, { as: 'creator' });
// Places creatorId on album rows
// Allows for artist.getAlbums/setAlbum(s)/addAlbum(s)/etc

User.belongsToMany(Song, { through: 'favorites' });
// Creates a new table in database named "favorites"
// with columns userId and songId
// This allows songs to be associated with many users
// and users to associate with many songs