How does Sequelize define models and relationships for a relational (SQL) database in Node.js?

Sequelize defines models as JavaScript classes mapped to database tables, with each column's type and constraints declared explicitly, and relationships (one-to-many, many-to-many, one-to-one) declared via methods like hasMany(), belongsTo(), and belongsToMany() -- Sequelize then handles generating the underlying SQL joins and foreign key management automatically when you query related data, and can also auto-generate migrations reflecting schema changes.

const { DataTypes } = require('sequelize');

const User = sequelize.define('User', {
  name: { type: DataTypes.STRING, allowNull: false },
  email: { type: DataTypes.STRING, unique: true },
});
const Post = sequelize.define('Post', { title: DataTypes.STRING });

User.hasMany(Post);
Post.belongsTo(User);

const userWithPosts = await User.findByPk(1, { include: Post });