Merge b4ce8ee into 5a670ef

docs/site/BelongsTo-relation.md

@@ -179,3 +179,42 @@ 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.
+
+## Notes
+### Recursive relation
+Given an e-commerce system has many `Category`, each `Category` may have several sub-categories, and may belong to 1 parent-category.
+```ts
+export class Category extends Entity {
+  @property({
+    type: 'number',
+    id: true,
+    generated: true
+  })
+  id?: number;
+
+  @belongsTo(() => Category)
+  parentId: number;
+
+  constructor(data?: Partial<Category>) {
+    super(data);
+  }
+}
+```
+
+The `CategoryRepository` must be declared like below
+```ts
+export class CategoryRepository extends DefaultCrudRepository<
+  Category,
+  typeof Category.prototype.id
+> {
+  public readonly parent: BelongsToAccessor<Category, number>;
+  constructor(
+    @inject('datasources.db') dataSource: DbDataSource
+  ) {
+    super(Category, dataSource);
+    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 https://github.com/strongloop/loopback-next/issues/2118)