docs(repository): has many relation docs

docs/site/Concepts.md

@@ -55,6 +55,9 @@ LoopBack 4 introduces some new concepts that are important to understand:
 - [**Repository**](Repositories.md): A type of service that represents a
   collection of data within a DataSource.
 
+- [**Relation**](Relations.md): A mapping between two models which describes a
+  real world link between them and exposes CRUD APIs based on the configuration.
+
 - [**Decorator**](Decorators.md): The pattern used to annotate or modify your
   class declarations and their members with metadata.
 

docs/site/Relations.md

@@ -0,0 +1,164 @@
+---
+lang: en
+title: 'Relations'
+keywords: LoopBack 4.0, LoopBack 4
+tags:
+sidebar: lb4_sidebar
+permalink: /doc/en/lb4/Relations.html
+summary:
+---
+
+## Overview
+
+Model relation in LoopBack 3 is one of its powerful features which help users
+define real-world mappings between their models, access sensible CRUD APIs for
+each of the models, and add querying and filtering capabilities for the relation
+APIs after scaffolding their LoopBack applications. In LoopBack 4, with the
+introduction of [repositories](Repositories.md), we aim to simplify the approach
+to relations by creating constrained repositories. This means that, based on the
+relation definition, certain constraints need to be honoured by the target model
+repository, and thus, we produce a constrained version of it as a navigational
+property on the source repository. The first relation type we have available is
+the `has many` relation which operates by constraining the target repository by
+the foreign key property on its associated model.
+
+## Defining a HasMany Relation
+
+Model relations in LoopBack 4 are defined by having a decorator on a designated
+relational property. The definition of the `has many` relation is done by using
+the `hasMany` decorator. The decorator takes in the target model class
+constructor and optionally a custom foreign key to store the relation metadata.
+The decorator logic also designates the relation type and tries to infer the
+foreign key on the target model (or `keyTo` in the relation metadata) as a
+sensible default value which is used in LoopBack 3. It also calls
+`property.array()` to ensure that the type of the property is inferred properly
+as an array of the target model instances. The decorated property name is used
+as the relation name and stored as part of the source model definition's
+relation metadata. The following example shows how to define a `hasMany`
+relation on a source model `Customer`.
+
+{% include code-caption.html content="/src/models/customer.model.ts" %}
+
+```ts
+import {Order} from './order.model.ts';
+import {Entity, property, hasmany} from '@loopback/repository';
+
+export class Customer extends Entity {
+  @property({
+    type: 'number',
+    id: true,
+  })
+  id: number;
+
+  @property({
+    type: 'string',
+  })
+  name: string;
+
+  @hasMany(Order) orders: Order[];
+}
+```
+
+The metadata generated for the above decoration is as follows:
+
+```js
+{
+  type: 2; // denotes hasMany relation
+  keyTo: customerId; // Order model should have this property declared
+}
+```
+
+The property type metadata is also preserved as an array of type `Order` as part
+of the decoration. Another usage of the decorator with a custom foreign key name
+for the above example is as follows:
+
+```ts
+// inside Customer model class
+@hasMany(Order, {keyTo: 'custId'}) orders: Order[];
+```
+
+## Configuring a HasMany relation
+
+Once a `HasMany` 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:
+
+- Use [Dependency Injectition](Dependency-injection.md) to inject an instance of
+  the target repository in the constructor of your source repository class.
+- Declare a property with the factory function type
+  `HasManyRepositoryFactory<TargetModel, typeof TargetModel.prototype.id>` on
+  the source repository class.
+- call the `_createHasManyRepositoryFactoryFor` function in the constructor of
+  the source repository class with the relation name (decorated relation propert
+  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:
+
+{% include code-caption.html
+content="/src/repositories/customer.repository.ts.ts" %}
+
+```ts
+{import} {Order} from '../models/order.model.ts';
+{import} {Customer} from '../models/customer.model.ts';
+{import} {OrderRepository} from './order.repository.ts';
+{import} {DefaultCrudRepository, juggler, HasManyRepositoryFactory} from '@loopback/repository';
+
+class CustomerRepository extends DefaultCrudRepository<
+  Customer,
+  typeof Customer.prototype.id
+> {
+  public orders: HasManyRepositoryFactory<Order, typeof Order.prototype.id>;
+  constructor(
+    @inject('datasources.db') protected db: juggler.DataSource,
+    @repository(OrderRepository) orderRepository: OrderRepository,
+  ) {
+    super(Customer, db);
+    this.orders = this._createHasManyRepositoryFactoryFor(
+      'orders',
+      orderRepository,
+    );
+  }
+}
+```
+
+The following CRUD APIs are now available in the constrained target repository
+factory `orders` for instances of `customerRepository`:
+
+- `create` for creating a target model instance
+- `find` finding target model instance(s)
+- `delete` for deleting target model instance(s)
+- `patch` for patching target model instance(s)
+
+## Using constrained repository in a controller
+
+Once the has many relation has been defined and configured, it can be accessed
+via controller methods to call the underlying constrained repository CRUD APIs
+and exposed in LoopBack's swagger UI once decorated with
+[Route decorators](Routes.md#using-route-decorators-with-controller-methods). It
+will require the value of the foreign key and, depending on the request method,
+a partial value for the target model instance as demonstrated below.
+
+{% include code-caption.html
+content="src/controllers/customer-orders.controller.ts" %}
+
+```ts
+import {post, param, requestBody} from '@loopback/rest';
+import {customerRepository} from '../repositories/customer.repository.ts';
+import {Order} from '../models/order.model.ts';
+
+export class CustomerOrdersController {
+  constructor(
+    @repository(CustomerRepository)
+    protected customerRepository: CustomerRepository,
+  ) {}
+
+  @post('/customers/{id}/order')
+  async createCustomerOrders(
+    @param.path.number('id') customerId: number,
+    @requestBody() orderData: Partial<Order>,
+  ): Promise<Order> {
+    return await this.customerRepository.orders(customerId).create(orderData);
+  }
+}
+```