medusa-modules

Medusa v2 Custom Modules

Before writing code

Fetch live docs:

1. Fetch https://docs.medusajs.com/learn/fundamentals/modules for module overview
2. Web-search site:docs.medusajs.com DML data model reference for model property types
3. Web-search site:docs.medusajs.com MedusaService methods for base service API
4. Web-search site:docs.medusajs.com module links for linking modules together
5. Web-search site:docs.medusajs.com module container registration for dependency injection

Module Concept

A module is a self-contained package of functionality in Medusa v2:

• Encapsulates data models, services, and business logic
• Registered in medusa-config.ts under the modules array
• Accessed from workflows, API routes, and subscribers via the dependency container
• Modules communicate through links, not direct imports

Module Directory Structure

File	Purpose
src/modules/my-module/models/my-entity.ts	DML data model
src/modules/my-module/service.ts	Service extending MedusaService
src/modules/my-module/loaders/seed.ts	Optional loader (runs on startup)
src/modules/my-module/migrations/	Auto-generated by CLI
src/modules/my-module/index.ts	Module definition export

DML Data Models

Property Types

DML Type	Database Column	Notes
model.id()	Primary key UUID	Auto-generated
model.text()	text / varchar	String fields
model.number()	integer	Numeric fields
model.bigNumber()	numeric	Precise decimals (prices)
model.boolean()	boolean	True/false
model.dateTime()	timestamptz	Date/time values
model.json()	jsonb	Arbitrary JSON
model.enum()	enum	Constrained values
model.array()	array	Array of primitives

Relationship Types

Relationship	DML Method	Description
Has One	model.hasOne()	One-to-one owned
Has Many	model.hasMany()	One-to-many owned
Belongs To	model.belongsTo()	Inverse of hasOne/hasMany
Many to Many	model.manyToMany()	Junction table auto-created

Model Definition Skeleton

// src/modules/my-module/models/my-entity.ts
// Fetch live docs for current DML model API
import { model } from "@medusajs/framework/utils"

const MyEntity = model.define("my_entity", {
  id: model.id(), name: model.text(),
  // Fetch live docs for property options and relationships
})

Service Pattern

Every module exposes a service that extends MedusaService:

// src/modules/my-module/service.ts
// Fetch live docs for MedusaService generic signature
import { MedusaService } from "@medusajs/framework/utils"
import MyEntity from "./models/my-entity"

class MyModuleService extends MedusaService({ MyEntity }) {}
export default MyModuleService // Add custom methods as needed

Inherited MedusaService Methods

Method Pattern	Purpose
create<Entity>(data)	Create one or many records
update<Entity>(data)	Update records by selector or ID
delete<Entity>(ids)	Soft-delete records
retrieve<Entity>(id, config)	Retrieve single record by ID
list<Entity>(filters, config)	List with filters, pagination, relations
softRestore<Entity>(ids)	Restore soft-deleted records

The method names are auto-generated from the model name (e.g., createMyEntity, listMyEntities).

Module Definition Export

// src/modules/my-module/index.ts
// Fetch live docs for Module definition shape
import { Module } from "@medusajs/framework/utils"
import MyModuleService from "./service"

export const MY_MODULE = Module("my-module", {
  service: MyModuleService,
})

Registration in medusa-config.ts

// Inside modules array
// Fetch live docs for module registration options
{ resolve: "./src/modules/my-module" }

Module Links

Links connect data models across different modules without coupling:

Link Type	Purpose
One-to-one link	Connect a custom entity to a core entity (e.g., product)
One-to-many link	One core entity linked to many custom entities
List link	Bidirectional many-to-many between modules

Link Definition Skeleton

// src/links/product-my-entity.ts
// Fetch live docs for defineLink API
import { defineLink } from "@medusajs/framework/utils"
import ProductModule from "@medusajs/medusa/product"
import { MY_MODULE } from "../modules/my-module"

Links are queried using the query utility from the Medusa container, not through direct service calls.

Loaders

Loaders run when the module initializes (server startup):

• Seed default data
• Register event listeners
• Validate configuration
• Set up external connections

Container Resolution

Modules register their service in the dependency injection container:

• Resolved by module key: container.resolve("my-module")
• Available in workflows, API routes, subscribers, and other services
• Never import a module's service directly -- always resolve from the container

Best Practices

• One module per bounded context -- keep modules focused and cohesive
• Use DML models exclusively -- never write raw SQL or use Mikro-ORM directly
• Extend MedusaService for CRUD -- only add custom methods for non-standard logic
• Use module links to connect to core entities -- never modify core module tables
• Run npx medusa db:generate after every model change to create migrations
• Export a constant for your module key (e.g., MY_MODULE) and reuse it everywhere

Common Patterns

Pattern	When to Use
Custom module + link to Product	Adding metadata/features to products
Custom module + link to Order	Order-related extensions (loyalty, tracking)
Standalone module	Independent domain (CMS, analytics, reviews)
Module with loader	Pre-seeding data or external service setup

Fetch the Medusa module documentation for exact DML property options, MedusaService generics, and link definition patterns before implementing.