[Spike] HasMany relation to same Entity

[jannyHou]

Description / Steps to reproduce / Feature proposal

This is a spike story for #1571. Based on the discussion in the original story, we need a quick spike to

• Create a test case to mock the scenario - Entity creates a hasMany relation to itself.
• See if it fails. If yes:
  • Troubleshoot the reason, e.g. infinite dereference of circular dependencies or other reasons.
  • Discover the gap between the potential fix and the current code.

Timebox it to 2 days

[danysz]

When we will have a fix for it?

[nabdelgadir]

I modified the Todo model by adding the following two properties:

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

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

And the Todo repository as follows:

import {
  DefaultCrudRepository,
  juggler,
  HasManyRepositoryFactory,
  BelongsToAccessor,
} from '@loopback/repository';
import {Todo} from '../models';
import {inject, Getter} from '@loopback/core';

export class TodoRepository extends DefaultCrudRepository<
  Todo,
  typeof Todo.prototype.id
> {
  // EDITED HERE
  public readonly todos: HasManyRepositoryFactory<
    Todo,
    typeof Todo.prototype.id
  >;
  public readonly parentId: BelongsToAccessor<Todo, typeof Todo.prototype.id>;

  constructor(@inject('datasources.db') dataSource: juggler.DataSource) {
    super(Todo, dataSource);
    // EDITED HERE
    this.parentId = this.createBelongsToAccessorFor(
      'parentId',
      Getter.fromValue(this),
    );
    this.todos = this.createHasManyRepositoryFactoryFor(
      'todos',
      Getter.fromValue(this),
    );
  }
}

And added the following two endpoints to the controller:

@post('/todos/{id}/todo')
async createTodo(
  @param.path.number('id') todoId: typeof Todo.prototype.id,
  @requestBody() todoData: Todo,
): Promise<Todo> {
  return await this.todoRepo.todos(todoId).create(todoData);
}

@get('/todos/{id}/parentId')
async getTodo(
  @param.path.number('id') todoId: typeof Todo.prototype.id,
): Promise<Todo> {
  return await this.todoRepo.parentId(todoId);
}

Then tried creating a parent Todo with only a title property [POST /todos]:

{
  "id": 5,
  "title": "parent"
}

Then tried adding a child Todo via POST /todos/{id}/todo:

{
  "id": 6,
  "title": "child",
  "parentId": 5
}

Then when I tried getting the parent Todo via GET /todos/6/parentId:

{
  "id": 5,
  "title": "parent"
}

However, if I try GET todos/5/, it doesn't include the child Todo (I assume this will be available when partial updates are supported), but if I try GET todos/6/ then I do get the parentId:

[result from GET /todos/]

{
  // ...
  {
    "id": 5,
    "title": "parent"
  },
  {
    "id": 6,
    "title": "child",
    "parentId": 5
  }
}

@b-admike thoughts? Did I miss anything?

[b-admike]

@nabdelgadir That looks good to me. Thank you for verifying! We can now capture this scenario in a test case in https://github.com/strongloop/loopback-next/blob/master/packages/repository/src/__tests__/acceptance/has-many.relation.acceptance.ts.

[nabdelgadir]

Going to close this spike as the error couldn't be reproduced and leave adding a test as part of the original issue #1571.