[Proposal] Seed feature

[emulienfou]

Loopback allow by default (using lb4-cli) to migrate database schema.
However, there is no script provided by default to seed database.

The current solution I found was this custom script: https://stackoverflow.com/a/54796872

Proposition

The idea would be to integrate by default (same as the migrate script) a feature to seed database depending on multiple sources of data (json files, manually create using repository create, ...).

Code

Here is an code I made using a custom Mixin, based on the idea of the next stackoverflow: Loopback 4: Create seeders to add dummy data in mySQL table.

The mixin

import { Application, MixinTarget } from '@loopback/core'
import debugFactory from 'debug'
import { DefaultCrudRepository, Entity } from '@loopback/repository'

const debug = debugFactory('loopback:seed:mixin')

export interface SeedMixinInterface {
  loadSeeds(): Promise<void>;
}

export function SeedMixin<T extends MixinTarget<Application>>(superClass: T) {
  return class extends superClass {
    async load(): Promise<void> {
      /* eslint-disable @typescript-eslint/ban-ts-comment */
      // A workaround to access protected Application methods
      const self = this as unknown as Application
      debug('Loading seeds')
      // @ts-ignore
      await self.loadSeeds()
    }

    async loadByModel<I extends Entity, ID>(items: I[], repository$: DefaultCrudRepository<I, ID>, type: { new(it: Partial<I>): I; }): Promise<void> {
      debug('Seeding data for model "%s"', type.name)
      /* eslint-disable-next-line @typescript-eslint/await-thenable */
      const repository = await repository$
      await repository.deleteAll()
      await Promise.all(items.map(async (item: I) => {
        try {
          return await repository.create((new type(item)))
        } catch (e) {
          debug('Error: %s', e.message)
        }
      }))
    }
  }
}

Application

This mixin needs to be implemented in the Application class as follow:

import { SeedMixin, SeedMixinInterface } from './mixins'

export class Application extends BootMixin(ServiceMixin(RepositoryMixin(SeedMixin(RestApplication)))) implements SeedMixinInterface {
   // ...

  async loadSeeds(): Promise<void> {
    // Implement your logic to load seeds here (e.g. JSON file, repository, ...)
    const models = [new Model()]
    await this.loadByModel(models, await this.getRepository(ModelRepository), Model)
  }
}

Seed script

The seed script located in: src/seed.ts

import { Application } from './application'

export const seed = async () => {
  const app = new Application()
  await app.boot()
  await app.load()

  // Connectors usually keep a pool of opened connections,
  // this keeps the process running even after all work is done.
  // We need to exit explicitly.
  process.exit(0)
}

seed().catch(err => {
  console.error('Cannot seed database schema', err)
  process.exit(1)
})

Updating package.json

Last step would be to declare the new seed script as follow:

{
  "scripts": {
    "preseed": "yarn run build",
    "seed": "node ./dist/seed",
  }
}