Overview

Sequelize is a mature, promise-based ORM for Node.js supporting PostgreSQL, MySQL, MariaDB, SQLite, and MS SQL Server. The stable line is v6 (sequelize 6.37.x on npm, with sequelize-cli 6.6.x for migrations). Version 7 is published as @sequelize/core but is still an alpha (7.0.0-alpha.x): it has a different package layout (one @sequelize/<dialect> package per database) and decorator-based models, so use v6 for production unless the user asks for v7. Docs: https://sequelize.org/docs/v6/.

Instructions

Core Capabilities

Model Definition

typescript

import { Model, DataTypes, Sequelize, QueryTypes, InferAttributes, InferCreationAttributes, CreationOptional } from "sequelize";

const sequelize = new Sequelize(process.env.DATABASE_URL!, {
  dialect: "postgres",
  pool: { max: 20, min: 5, acquire: 30000, idle: 10000 },
  logging: process.env.NODE_ENV === "development" ? console.log : false,
});

class User extends Model<InferAttributes<User>, InferCreationAttributes<User>> {
  declare id: CreationOptional<number>;
  declare name: string;
  declare email: string;
  declare role: CreationOptional<"user" | "admin">;
  declare createdAt: CreationOptional<Date>;
  declare updatedAt: CreationOptional<Date>;
}

User.init({
  id: { type: DataTypes.INTEGER, autoIncrement: true, primaryKey: true },
  name: { type: DataTypes.STRING(100), allowNull: false, validate: { len: [2, 100] } },
  email: { type: DataTypes.STRING, allowNull: false, unique: true, validate: { isEmail: true } },
  role: { type: DataTypes.ENUM("user", "admin"), defaultValue: "user" },
}, {
  sequelize, tableName: "users", timestamps: true,
  hooks: {
    beforeCreate: (user) => { user.email = user.email.toLowerCase(); },
  },
});

class Post extends Model<InferAttributes<Post>, InferCreationAttributes<Post>> {
  declare id: CreationOptional<number>;
  declare title: string;
  declare body: string;
  declare published: CreationOptional<boolean>;
  declare authorId: number;
}

Post.init({
  id: { type: DataTypes.INTEGER, autoIncrement: true, primaryKey: true },
  title: { type: DataTypes.STRING, allowNull: false },
  body: { type: DataTypes.TEXT, allowNull: false },
  published: { type: DataTypes.BOOLEAN, defaultValue: false },
  authorId: { type: DataTypes.INTEGER, allowNull: false },
}, { sequelize, tableName: "posts" });

// Associations
User.hasMany(Post, { foreignKey: "authorId", as: "posts" });
Post.belongsTo(User, { foreignKey: "authorId", as: "author" });

Queries

typescript

// Find with eager loading
const users = await User.findAll({
  where: { role: "user" },
  include: [{ model: Post, as: "posts", where: { published: true }, required: false }],
  order: [["createdAt", "DESC"]],
  limit: 10, offset: 20,
});

// Raw query for complex operations; pass values with replacements/bind, never string concatenation
const rows = await sequelize.query(`
  SELECT u.name, COUNT(p.id) AS post_count
  FROM users u LEFT JOIN posts p ON u.id = p."authorId" AND p.published = :published
  GROUP BY u.id ORDER BY post_count DESC LIMIT 10
`, { replacements: { published: true }, type: QueryTypes.SELECT });

// Transaction
// Managed transaction: commits on return, rolls back on throw
await sequelize.transaction(async (t) => {
  const user = await User.create({ name: "Alice Moreau", email: "alice@northwind.io" }, { transaction: t });
  await Post.create({ title: "First Post", body: "Hello", authorId: user.id }, { transaction: t });
});

// Bulk operations
await User.bulkCreate(usersData, { validate: true, updateOnDuplicate: ["name"] });   // upsert on the unique key (not supported on every dialect)

Installation

bash

npm install sequelize
npm install pg pg-hstore                  # PostgreSQL (or mysql2, mariadb, sqlite3, tedious)
npm install --save-dev sequelize-cli      # Migrations CLI
npx sequelize-cli init                    # Creates config/, models/, migrations/, seeders/

Migrations with the CLI (npx sequelize-cli ...; the default config/config.json is created with mysql, so edit dialect and credentials first):

bash

npx sequelize-cli model:generate --name Task --attributes title:string,done:boolean
npx sequelize-cli db:migrate              # apply pending migrations
npx sequelize-cli db:migrate:status
npx sequelize-cli db:migrate:undo         # revert the last one

Examples

Example 1: Blog backend with users and posts

User request: "Set up Sequelize with Postgres for a blog: users have many posts, I want the latest 10 users with their published posts."

The agent installs sequelize pg pg-hstore, creates the User and Post models and the hasMany/belongsTo pair above, generates migrations with sequelize-cli instead of sync(), and runs User.findAll({ include: [{ model: Post, as: "posts", where: { published: true }, required: false }], order: [["createdAt", "DESC"]], limit: 10 }). Result: an array of User instances, each with a posts array (empty when the author has nothing published).

Example 2: Add a column safely

User request: "Add a nullable bio text column to users in production without losing data."

The agent runs npx sequelize-cli migration:generate --name add-bio-to-users, fills in up with queryInterface.addColumn("users", "bio", { type: Sequelize.TEXT, allowNull: true }) and down with queryInterface.removeColumn("users", "bio"), adds declare bio: string | null to the model, then runs db:migrate. A failed migration can be reverted with db:migrate:undo.

Guidelines

Migrations, not sync() — use sequelize-cli migrations in production; sync({ alter: true }) and sync({ force: true }) can drop or rewrite data
TypeScript — use InferAttributes / InferCreationAttributes with declare fields and CreationOptional for generated columns; do not use class field initializers (they shadow Sequelize getters)
Scopes — define reusable filters with defaultScope/scopes and call User.scope('active').findAll()
Transactions — managed transactions (callback form) roll back automatically; pass { transaction: t } to every query in them, or enable CLS (npm install cls-hooked, then Sequelize.useCLS(namespace)) for automatic propagation
Paranoid mode — paranoid: true gives soft deletes through a deletedAt column
Eager loading — include loads associations; required: false gives a LEFT JOIN, and large nested includes with limit can produce heavy queries (use separate: true for hasMany)
Hooks — beforeCreate/afterUpdate are skipped by bulkCreate/update unless individualHooks: true
Connection pool — set pool.max below the database's connection limit divided by app instances
SQL injection — never interpolate user input into sequelize.query; use replacements or bind

Sequelize

Validation

Getting Started

Example Prompts

Documentation