Sequelize がモデルの関連付けに外部キー列を作成しないのはなぜですか?

// database.js

// Register all models in one place
const fs = require('fs');
const path = require('path');
const Sequelize = require('sequelize');
const db = {};

const models = path.join(__dirname, 'models');

// Create a Sequelize instance
const sequelize = new Sequelize(/* your connection settings here */);

// Register models and add them to the db object
fs.
  readdirSync(models).
  filter(function (file) {
    return (file.indexOf('.') !== 0) && (file !== basename) && (file.slice(-3) === '.js');
  }).
  forEach(function (file) {
    // Sequelize version <= 5.x
    var model = sequelize['import'](path.join(models, file));
    // Sequelize version >= 6.x
    // var model = require(path.join(models, file))(
    //   sequelize,
    //   Sequelize.DataTypes
    // );
    db[model.name] = model;
  });

// Associate models within the db object
Object.keys(db).forEach(function (modelName) {
  if (db[modelName].associate) {
    db[modelName].associate(db);
  }
});

db.Sequelize = Sequelize; // for accessing static props and functions like Op.or
db.sequelize = sequelize; // for accessing connection props and functions like 'query' or 'transaction'

module.exports = db;

const db = require('../database');
const { Op } = require('sequelize');

// Use models defined in database.js
const users = await db.user.findAll({
  where: {
    [Op.or]: [
      {
        first_name: 'Smith'
      },
      {
        last_name: 'Smith'
      }
    ]
  }
});