How to migrate CRUD Operation Hooks

[bajtos]

This is a follow-up for #3718 and #3922.

Write content for docs/site/migration/models/operation-hooks.md, explain how to migrate operation hooks (see LB3 docs: Operation Hooks).

Explain that we don't have first-class support for Operation Hooks in LB4 yet, refer to #1919

As a temporary solution, describe how to leverage PersistedModel used by our legacy juggler bridge. Example implementation:

export class TodoRepository extends DefaultCrudRepository<
  Todo,
  typeof Todo.prototype.id,
  TodoRelations
> {
  constructor(@inject('datasources.db') dataSource: juggler.DataSource) {
    super(Todo, dataSource);

    this.modelClass.observe('save', async ctx => {
      console.log(
        'Going to write to an instance of model %s',
        ctx.Model.modelName,
      );
    });
  }
}

Caveats:

(1)
Repository constructor is executed once for each incoming request. We don't want to install another copy of the same operation hook on each request, we need to find a way how to install hooks only once. Ideally, DefaultCrudRepository should provide a protected method that subclasses can use to supply one-time model setup.

For example:

export class TodoRepository extends DefaultCrudRepository<
  Todo,
  typeof Todo.prototype.id,
  TodoRelations
> {
  constructor(@inject('datasources.db') dataSource: juggler.DataSource) {
    super(Todo, dataSource);
  }
  
  protected setupPersistedModel() {
    super.setupPersistedModel();

    this.modelClass.observe('save', async ctx => {
      console.log(
        'Going to write to an instance of model %s',
        ctx.Model.modelName,
      );
    });
  }
}

(2)
Current TypeScript typings in loopback-datasource-juggler do not contain operation hooks and .observe() method. We need to fix this.

Acceptance criteria

• Add typings for Operation Hooks to loopback-datasource-juggler
• Add a protected method definePersistedModel to DefaultCrudRepository, this method should overriden by user's repository class to access the model class where the operation hooks can be applied.
• Migration guide describing how to move LB3 operation hooks to LB4 repositories.

[marioestradarosa]

@bajtos

How about decorators inside the repository to allow a particular method to become part of the event (before/after etc)

@observe('before-save')
async myMethodInsideRepository(rec: MyModel) : Promise<MyModel> {

}

[hacksparrow]

@bajtos how about this alternate interface?

// repo is a Repository instance
repo.observe('before save', async ctx => {
  console.log(
    'Going to write to an instance of model %s',
    ctx.Model.modelName,
  );
});

The hooks are not hard-coded in the repository class, you add them as required using an instance method.

[bajtos]

@hacksparrow can you please clarify how is your interface dealing with the following problem?

Repository constructor is executed once for each incoming request. We don't want to install another copy of the same operation hook on each request, we need to find a way how to install hooks only once

Also IMO, setup of operation hooks is a responsibility of per-model repository class (e.g. ProductRepository), it's not a responsibility of repository consumers (controllers, etc.).