3.5.4. Data Model Relationships

In this chapter, you’ll learn how to define relationships between data models in your module.

What is a Relationship Property?#

A relationship property defines an association in the database between two models. It's created using the Data Model Language (DML) methods, such as hasOne or belongsTo.

When you generate a migration for these data models, the migrations include foreign key columns or pivot tables, based on the relationship's type.

One-to-One Relationship#

A one-to-one relationship indicates that one record of a data model belongs to or is associated with another.

To define a one-to-one relationship, create relationship properties in the data models using the following methods:

1. hasOne: indicates that the model has one record of the specified model.
2. belongsTo: indicates that the model belongs to one record of the specified model.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3const User = model.define("user", {4  id: model.id().primaryKey(),5  email: model.hasOne(() => Email, {6    mappedBy: "user",7  }),8})9
10const Email = model.define("email", {11  id: model.id().primaryKey(),12  user: model.belongsTo(() => User, {13    mappedBy: "email",14  }),15})

In the example above, a user has one email, and an email belongs to one user.

The hasOne and belongsTo methods accept a function as the first parameter. The function returns the associated data model.

Both methods also accept a second parameter object with the property mappedBy. Its value is the name of the relationship property in the other data model.

Optional Relationship#

To make the relationship optional on the hasOne or belongsTo side, use the nullable method on either property as explained in this chapter.

One-to-One Relationship in the Database#

When you generate the migrations of data models that have a one-to-one relationship, the migration adds to the table of the data model that has the belongsTo property:

1. A column of the format {relation_name}_id to store the ID of the record of the related data model. For example, the email table will have a user_id column.
2. A foreign key on the {relation_name}_id column to the table of the related data model.

One-sided One-to-One Relationship#

In some use cases, you may want to define a one-to-one relationship only on one side. This means that the other data model does not have a relationship property pointing to the first one.

You can do this either from the hasOne or the belongsTo side.

hasOne Side

By default, the foreign key column is added to the table of the data model that has the belongsTo property. For example, if the Email data model belongs to the User data model, then the foreign key column is added to the email table.

If you want to define a one-to-one relationship only on the User data model's side (hasOne side), you can do so by passing the following properties to the second parameter of the hasOne method:

• foreignKey: A boolean indicating whether the foreign key column should be added to the table of the data model.
• mappedBy: Set to undefined, since the relationship is only defined on one side.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3const User = model.define("user", {4  id: model.id().primaryKey(),5  email: model.hasOne(() => Email, {6    foreignKey: true,7    mappedBy: undefined,8  }),9})10
11const Email = model.define("email", {12  id: model.id().primaryKey(),13})

In the example above, you add a one-to-one relationship from the User data model to the Email data model.

The foreign key column is added to the user table, and the Email data model does not have a relationship property pointing to the User data model.

belongsTo Side

To define the one-to-one relationship on the belongsTo side, pass undefined to the mappedBy property in the belongsTo method's second parameter.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3const User = model.define("user", {4  id: model.id().primaryKey(),5})6
7const Email = model.define("email", {8  id: model.id().primaryKey(),9  user: model.belongsTo(() => User, {10    mappedBy: undefined,11  }),12})

In the example above, you add a one-to-one relationship from the Email data model to the User data model.

The User data model does not have a relationship property pointing to the Email data model.

One-to-Many Relationship#

A one-to-many relationship indicates that one record of a data model has many records of another data model.

To define a one-to-many relationship, create relationship properties in the data models using the following methods:

1. hasMany: indicates that the model has more than one record of the specified model.
2. belongsTo: indicates that the model belongs to one record of the specified model.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3const Store = model.define("store", {4  id: model.id().primaryKey(),5  products: model.hasMany(() => Product, {6    mappedBy: "store",7  }),8})9
10const Product = model.define("product", {11  id: model.id().primaryKey(),12  store: model.belongsTo(() => Store, {13    mappedBy: "products",14  }),15})

In this example, a store has many products, but a product belongs to one store.

Optional Relationship#

To make the relationship optional on the belongsTo side, use the nullable method on the property as explained in this chapter.

One-to-Many Relationship in the Database#

When you generate the migrations of data models that have a one-to-many relationship, the migration adds to the table of the data model that has the belongsTo property:

1. A column of the format {relation_name}_id to store the ID of the record of the related data model. For example, the product table will have a store_id column.
2. A foreign key on the {relation_name}_id column to the table of the related data model.

Many-to-Many Relationship#

A many-to-many relationship indicates that many records of a data model can be associated with many records of another data model.

To define a many-to-many relationship, create relationship properties in the data models using the manyToMany method.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3const Order = model.define("order", {4  id: model.id().primaryKey(),5  products: model.manyToMany(() => Product, {6    mappedBy: "orders",7    pivotTable: "order_product",8    joinColumn: "order_id",9    inverseJoinColumn: "product_id",10  }),11})12
13const Product = model.define("product", {14  id: model.id().primaryKey(),15  orders: model.manyToMany(() => Order, {16    mappedBy: "products",17  }),18})

The manyToMany method accepts two parameters:

1. A function that returns the associated data model.
2. An object of optional configuration. Only one of the data models in the relation can define the pivotTable, joinColumn, and inverseJoinColumn configurations, and it's considered the owner data model. The object can accept the following properties:
   • mappedBy: The name of the relationship property in the other data model. If not set, the property's name is inferred from the associated data model's name.
   • pivotTable: The name of the pivot table created in the database for the many-to-many relation. If not set, the pivot table is inferred by combining the names of the data models' tables in alphabetical order, separating them by _, and pluralizing the last name. For example, order_products.
   • joinColumn: The name of the column in the pivot table that points to the owner model's primary key.
   • inverseJoinColumn: The name of the column in the pivot table that points to the owned model's primary key.

In this example, an order is associated with many products, and a product is associated with many orders. Since the pivotTable, joinColumn, and inverseJoinColumn configurations are defined on the order, it's considered the owner data model.

Many-to-Many Relationship in the Database#

When you generate the migrations of data models that have a many-to-many relationship, the migration adds a new pivot table. Its name is either the name you specify in the pivotTable configuration or the inferred name combining the names of the data models' tables in alphabetical order, separating them by _, and pluralizing the last name. For example, order_products.

The pivot table has a column with the name {data_model}_id for each of the data model's tables. It also has foreign keys on each of these columns to their respective tables.

The pivot table has columns with foreign keys pointing to the primary key of the associated tables. The column's name is either:

• The value of the joinColumn configuration for the owner table, and the inverseJoinColumn configuration for the owned table;
• Or the inferred name {table_name}_id.

Many-To-Many with Custom Columns#

To add custom columns to the pivot table between two data models having a many-to-many relationship, you must define a new data model that represents the pivot table.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3export const Order = model.define("order_test", {4  id: model.id().primaryKey(),5  products: model.manyToMany(() => Product, {6    pivotEntity: () => OrderProduct,7  }),8})9
10export const Product = model.define("product_test", {11  id: model.id().primaryKey(),12  orders: model.manyToMany(() => Order),13})14
15export const OrderProduct = model.define("orders_products", {16  id: model.id().primaryKey(),17  order: model.belongsTo(() => Order, {18    mappedBy: "products",19  }),20  product: model.belongsTo(() => Product, {21    mappedBy: "orders",22  }),23  metadata: model.json().nullable(),24})

The Order and Product data models have a many-to-many relationship. To add extra columns to the created pivot table, you pass a pivotEntity option to the products relation in Order (since Order is the owner). The value of pivotEntity is a function that returns the data model representing the pivot table.

The OrderProduct model defines, aside from the ID, the following properties:

• order: A relation that indicates this model belongs to the Order data model. You set the mappedBy option to the many-to-many relation's name in the Order data model.
• product: A relation that indicates this model belongs to the Product data model. You set the mappedBy option to the many-to-many relation's name in the Product data model.
• metadata: An extra column to add to the pivot table of type json. You can add other columns as well to the model.

Cascades#

When an operation is performed on a data model, such as record deletion, the relationship cascade specifies what related data model records should be affected by it.

For example, if a store is deleted, its products should also be deleted.

The cascades method used on a data model configures which child records an operation is cascaded to.

For example:

Code
1import { model } from "@medusajs/framework/utils"2
3const Store = model.define("store", {4  id: model.id().primaryKey(),5  products: model.hasMany(() => Product),6})7.cascades({8  delete: ["products"],9})10
11const Product = model.define("product", {12  id: model.id().primaryKey(),13  store: model.belongsTo(() => Store, {14    mappedBy: "products",15  }),16})

The cascades method accepts an object. Its key is the operation’s name, such as delete. The value is an array of relationship property names that the operation is cascaded to.

In the example above, when a store is deleted, its associated products are also deleted.