Generate Sequelize hasMany from SQL Foreign Key Relationships

hasMany: The Reverse Side of belongsTo

Every belongsTo relationship has a corresponding hasMany (or hasOne) on the other side. When the converter detects a foreign key from table B to table A, it generates both:

1. B.belongsTo(A) — on the child model
2. A.hasMany(B) — as a commented hint in the reverse associations section

Example: Users and Posts

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

CREATE TABLE posts (
  id SERIAL PRIMARY KEY,
  title VARCHAR(255) NOT NULL,
  author_id INTEGER NOT NULL REFERENCES users(id)
);

The converter produces:

// On the Post model:
// Post.belongsTo(User, { foreignKey: 'author_id', targetKey: 'id' });

// In the reverse associations section:
// User.hasMany(Post, { foreignKey: 'author_id' });

Why Comments Instead of Executable Code?

The converter generates hasMany as comments because:

1. Circular dependencies: In a modular project, model files import each other. Placing both association sides in the model files creates circular imports.
2. Association setup pattern: Best practice is to call associations in a central models/index.js or a dedicated associations.js file after all models are loaded.
3. Flexibility: You may want hasOne instead of hasMany, or you may want to add as aliases, onDelete, and onUpdate options.

Setting Up Both Sides

// models/associations.js
const { User, Post, Comment } = require('./');

User.hasMany(Post, { foreignKey: 'author_id', as: 'posts' });
Post.belongsTo(User, { foreignKey: 'author_id', as: 'author' });

Post.hasMany(Comment, { foreignKey: 'post_id', as: 'comments' });
Comment.belongsTo(Post, { foreignKey: 'post_id' });

Eager Loading

Once both sides are set up, you can use Sequelize's include option for eager loading:

const user = await User.findByPk(1, {
  include: [{ model: Post, as: 'posts' }]
});