# Sequelize — Feature-Rich Node.js ORM for SQL Databases

> A mature promise-based ORM for PostgreSQL, MySQL, MariaDB, SQLite, and SQL Server with transactions, relations, eager loading, and CLI migrations.

## Install

Save in your project root:

# Sequelize — Feature-Rich Node.js ORM for SQL Databases

## Quick Use
```bash
npm install sequelize pg pg-hstore
npx sequelize-cli init
npx sequelize-cli db:migrate
```

## Introduction
Sequelize is one of the most established ORMs in the Node.js ecosystem, providing a solid abstraction layer over SQL databases since 2011. It offers a promise-based API with comprehensive support for transactions, associations, and schema migrations.

## What Sequelize Does
- Provides model definitions that map JavaScript objects to SQL tables
- Supports PostgreSQL, MySQL, MariaDB, SQLite, and Microsoft SQL Server
- Handles one-to-one, one-to-many, and many-to-many associations with eager and lazy loading
- Offers a migration framework via sequelize-cli for version-controlled schema changes
- Includes built-in validation, hooks, and scopes for business logic

## Architecture Overview
Sequelize creates a connection pool per database instance using the underlying driver. Models are defined as classes extending Model, with attributes mapped to columns. The QueryInterface generates dialect-specific SQL, while the QueryGenerator handles parameterization. Associations are resolved at query time through JOIN clauses or separate queries for eager loading.

## Self-Hosting & Configuration
- Install sequelize plus the driver for your database (pg, mysql2, tedious, or better-sqlite3)
- Initialize project structure with sequelize-cli init to create config, models, migrations, and seeders directories
- Configure database credentials in config/config.json or use environment variables with .sequelizerc
- Set dialect options like pool size, SSL, and timezone in the Sequelize constructor
- Use migration files for all production schema changes; seeders for test data

## Key Features
- Mature ecosystem with 13+ years of production use and extensive documentation
- Raw query support with model mapping for complex SQL
- Transaction support with automatic retry and CLS-based propagation
- Paranoid mode for soft deletes with automatic filtering
- Comprehensive validation using validator.js integration

## Comparison with Similar Tools
- **TypeORM** — decorator-based, TypeScript-first; Sequelize is more JavaScript-idiomatic
- **Prisma** — schema file with generated client; Sequelize uses runtime model definitions
- **Knex.js** — query builder only; Sequelize adds full ORM layer with associations
- **Objection.js** — built on Knex with JSON Schema; Sequelize has its own query engine
- **Drizzle ORM** — newer, SQL-like API; Sequelize offers a more traditional ORM experience

## FAQ
**Q: Is Sequelize still actively maintained?**
A: Yes. Sequelize v7 is in active development with ESM support and improved TypeScript types.

**Q: How does Sequelize handle TypeScript?**
A: v6 supports TypeScript through manual type declarations. v7 improves this with better generics and InferAttributes helpers.

**Q: Can I use raw SQL with Sequelize?**
A: Yes. Use sequelize.query() with replacements or bind parameters, and optionally map results to model instances.

**Q: How do I optimize N+1 query problems?**
A: Use eager loading with the include option to fetch associations in a single JOIN query instead of separate queries.

## Sources
- https://github.com/sequelize/sequelize
- https://sequelize.org

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Author: AI Open Source