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Overview

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.

/src/models/order.model.ts

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

@model()
export class Order extends Entity {
  @property({
    type: 'number',
    id: true,
  })
  id: number;

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

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

  constructor(data: Partial<Order>) {
    super(data);
  }
}

export interface OrderRelations {
  // describe navigational properties here
}

export type OrderWithRelations = Order & OrderRelations;

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: number;
}

export interface OrderRelations {
  customer?: CustomerWithRelations;
}

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, accepting a string repository name instead of a repository constructor, or 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 sourceModel.prototype.id> 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:

/src/repositories/order.repository.ts

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

export class OrderRepository extends DefaultCrudRepository<
  Order,
  typeof Order.prototype.id,
  OrderRelations
> {
  public readonly customer: BelongsToAccessor<
    Customer,
    typeof Order.prototype.id
  >;

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

BelongsToAccessor is a function accepting the primary key (id) of a source model instance (e.g. order.id) 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.

src/controllers/order.controller.ts

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

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

  // (skipping regular CRUD methods for Order)

  @get('/orders/{id}/customer')
  async getCustomer(
    @param.path.number('id') orderId: typeof Order.prototype.id,
  ): Promise<Customer> {
    return 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.

Handling recursive relations

Given an e-commerce system has many Category, each Category may have several sub-categories, and may belong to 1 parent-category.

export class Category extends Entity {
  @property({
    type: 'number',
    id: true,
    generated: true,
  })
  id?: number;

  @hasMany(() => Category, {keyTo: 'parentId'})
  categories?: Category[];

  @belongsTo(() => Category)
  parentId?: number;

  constructor(data?: Partial<Category>) {
    super(data);
  }
}

export interface CategoryRelations {
  categories?: CategoryWithRelations[];
  parent?: CategoryWithRelations;
}

export type CategoryWithRelations = Category & CategoryRelations;

The CategoryRepository must be declared like below

export class CategoryRepository extends DefaultCrudRepository<
  Category,
  typeof Category.prototype.id,
  CategoryRelations
> {
  public readonly parent: BelongsToAccessor<
    Category,
    typeof Category.prototype.id
  >;
  public readonly categories: HasManyRepositoryFactory<
    Category,
    typeof Category.prototype.id
  >;

  constructor(@inject('datasources.db') dataSource: DbDataSource) {
    super(Category, dataSource);
    this.categories = this.createHasManyRepositoryFactoryFor(
      'categories',
      Getter.fromValue(this),
    );
    this.parent = this.createBelongsToAccessorFor(
      'parent',
      Getter.fromValue(this),
    ); // for recursive relationship
  }
}

DO NOT declare @repository.getter(CategoryRepository) protected categoryRepositoryGetter: Getter<CategoryRepository> on constructor to avoid “Circular dependency” error (see issue #2118)

A belongsTo relation has an inclusionResolver function as a property. It fetches target models for the given list of source model instances.

Use the relation between Customer and Order we show above, an Order belongs to a Customer.

After setting up the relation in the repository class, the inclusion resolver allows users to retrieve all orders along with their related customers through the following code:

orderRepo.find({include: [{relation: 'customer'}]});

Enable/disable the inclusion resolvers:

  • Base repository classes have a public property inclusionResolvers, which maintains a map containing inclusion resolvers for each relation.
  • The inclusionResolver of a certain relation is built when the source repository class calls the createBelongsToAccessorFor function in the constructor with the relation name.
  • Call registerInclusionResolver to add the resolver of that relation to the inclusionResolvers map. (As we realized in LB3, not all relations are allowed to be traversed. Users can decide to which resolvers can be added.) The first parameter is the name of the relation.

The following code snippet shows how to register the inclusion resolver for the belongsTo relation ‘customer’:

export class OrderRepository extends DefaultCrudRepository {
  customer: BelongsToAccessor<Customer, typeof Order.prototype.id>;

  constructor(
    dataSource: juggler.DataSource,
    customerRepositoryGetter: Getter<CustomerRepository>,
  ) {
    super(Order, dataSource);

    // we already have this line to create a BelongsToRepository factory
    this.customer = this.createBelongsToAccessorFor(
      'customer',
      customerRepositoryGetter,
    );

    // add this line to register inclusion resolver.
    this.registerInclusionResolver('customer', this.customer.inclusionResolver);
  }
}
  • We can simply include the relation in queries via find(), findOne(), and findById() methods. Example:

    orderRepository.find({include: [{relation: 'customer'}]});
    

    which returns:

    [
      {
        id: 1,
        description: 'Mjolnir',
        customerId: 1,
        customer: {
          id: 12,
          name: 'Thor',
        },
      },
      {
        id: 2,
        description: 'Shield',
        customer: {
          id: 10,
          name: 'Captain',
        },
      },
      {
        id: 3,
        description: 'Rocket Raccoon',
        customerId: 1,
        customer: {
          id: 12,
          name: 'Thor',
        },
      },
    ];
    
  • You can delete a relation from inclusionResolvers to disable the inclusion for a certain relation. e.g orderRepository.inclusionResolvers.delete('customer')