Add examples of associations

[papb]

The new example does not show any associations; it would be very useful to show associations being performed as well as suggesting how to organize the code for that.

See https://github.com/sequelize/express-example/issues/95#issuecomment-660696270

[NicholasEwing]

@papb Just trying to learn more about your code: May I ask why you used a modelDefiner function on each of these models? Why not just define the models in their file and import them like the following?

Your example:

const modelDefiners = [
    require('./models/user.model'),
    require('./models/instrument.model'),
    require('./models/orchestra.model'),
    // Add more models here...
    // require('./models/item'),
];

My example:

// top of file
const user = require('./models/user.model');
const instrument = require('./models/instrument.model');
const orchestra = require('./models/orchesta.model');

Inside of a model:

const instrument = sequelize.define('instrument', {
    // The following specification of the 'id' attribute could be omitted
    // since it is the default.
    id: {
        allowNull: false,
        autoIncrement: true,
        primaryKey: true,
        type: DataTypes.INTEGER
    },
    type: {
        allowNull: false,
        type: DataTypes.STRING,
    },
    // type: {
    //  allowNull: false,
    //  type: DataTypes.STRING,
    //  validate: {
    //      isIn: [['string', 'wind', 'percussion']]
    //  }
    // },
    purchaseDate: {
        allowNull: false,
        type: DataTypes.DATE
    },
    // We also want it to have a 'orchestraId' field, but we don't have to define it here.
    // It will be defined automatically when Sequelize applies the associations.

    module.exports = instrument;
});

What's the advantage of the model files being functions that take sequelize as a parameter? Just trying to learn more!