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A belongsTo relation denotes a many-to-one connection of a model to another model through referential integrity. The referential integrity is enforced by a foreign key constraint on the source model which usually references a primary key on the target model. This relation indicates that each instance of the declaring or source model belongs to exactly one instance of the target model. For example, in an application with customers and orders, an order always belongs to exactly one customer as illustrated in the diagram below.

belongsTo relation illustration

The diagram shows the declaring (source) model Order has property customerId as the foreign key to reference the target model Customer’s primary key id.

To add a belongsTo relation to your LoopBack application and expose its related routes, you need to perform the following steps:

  1. Add a property to your source model to define the foreign key.
  2. Modify the source model repository class to provide an accessor function for obtaining the target model instance.
  3. Call the accessor function in your controller methods.

Defining a belongsTo Relation

This section describes how to define a belongsTo relation at the model level using the @belongsTo decorator to define the constraining foreign key.


import {belongsTo, Entity, model, property} from '@loopback/repository';
import {Customer} from './customer.model';

export class Order extends Entity {
    type: 'number',
    id: true,
  id: number;

  @belongsTo(() => Customer)
  customerId: string;

  @property({type: 'number'})
  quantity: number;

  constructor(data: Partial<Order>) {

The definition of the belongsTo relation is inferred by using the @belongsTo decorator. The decorator takes in a function resolving the target model class constructor and designates the relation type. It also calls property() to ensure that the decorated property is correctly defined.

A usage of the decorator with a custom primary key of the target model for the above example is as follows:

class Order extends Entity {
  // constructor, properties, etc.
  @belongsTo(() => Customer, {keyTo: 'pk'})
  customerId: string;

Configuring a belongsTo relation

The configuration and resolution of a belongsTo relation takes place at the repository level. Once belongsTo relation is defined on the source model, then there are a couple of steps involved to configure it and use it. On the source repository, the following are required:

  • In the constructor of your source repository class, use Dependency Injection to receive a getter function for obtaining an instance of the target repository. Note: We need a getter function instead of a repository instance in order to break a cyclic dependency between a repository with a belongsTo relation and a repository with the matching hasMany relation.
  • Declare a property with the factory function type BelongsToAccessor<targetModel, typeof> on the source repository class.
  • call the _createBelongsToAccessorFor function in the constructor of the source repository class with the relation name (decorated relation property on the source model) and target repository instance and assign it the property mentioned above.

The following code snippet shows how it would look like:


import {Getter, inject} from '@loopback/context';
import {
} from '@loopback/repository';
import {Customer, Order} from '../models';
import {CustomerRepository} from '../repositories';

export class OrderRepository extends DefaultCrudRepository<
> {
  public readonly customer: BelongsToAccessor<

    @inject('datasources.db') protected db: juggler.DataSource,
    customerRepositoryGetter: Getter<CustomerRepository>,
  ) {
    super(Order, db);
    this.customer = this._createBelongsToAccessorFor(

BelongsToAccessor is a function accepting the primary key (id) of a source model instance (e.g. and returning back the related target model instance (e.g. a Customer the order belongs to). See also API Docs

Using BelongsToAccessor in a controller

The same pattern used for ordinary repositories to expose their CRUD APIs via controller methods is employed for belongsTo relation too. Once the belongsTo relation has been defined and configured, a new controller method can expose the accessor API as a new endpoint.


import {repository} from '@loopback/repository';
import {get} from '@loopback/rest';
import {Customer, Order} from '../models/';
import {OrderRepository} from '../repositories/';

export class OrderController {
    @repository(OrderRepository) protected orderRepository: OrderRepository,
  ) {}

  // (skipping regular CRUD methods for Order)

  async getCustomer(
    @param.path.number('id') orderId: typeof,
  ): Promise<Customer> {
    return await this.orderRepository.customer(orderId);

In LoopBack 3, the REST APIs for relations were exposed using static methods with the name following the pattern __{methodName}__{relationName}__ (e.g. Order.__get__customer). While we recommend to create a new controller for each hasMany relation in LoopBack 4, we also think it’s best to use the main CRUD controller as the place where to explose belongsTo API.