NestJS MergePipe Utility

MergePipeTransform.ts

import {
  ArgumentMetadata,
  Injectable,
  mixin,
  PipeTransform,
  Type,
} from '@nestjs/common'
import { ModuleRef } from '@nestjs/core'

/**
 * Takes multiple pipe in a map and apply them to the same argument.
 * Example:
 *     Assuming you have 3 models, User has many Book and Post: 
 *     Instead of having 3 pipes, you can now use one MergePipe to extract all 3 pieces of information.
 * 
 *     @CurrentUserId(IdToUserTransform, MergePipe({  
 *         books: UserToBooksTransform,
 *         posts: UserToPostsTransform
 *     }) extracted: {
 *         books: Book[]
 *         posts: Post[],
 *         identity: User
 *     }
 */
@Injectable()
export class MergePipeTransform implements PipeTransform {
  protected readonly transformIdentity: boolean
  protected readonly transformMap: { [key: string]: Type<PipeTransform> }

  constructor(private readonly moduleRef: ModuleRef) {}

  async transform(
    val: any,
    metadata: ArgumentMetadata
  ): Promise<{ [key: string]: any }> {
    const result = {}

    for (const key in this.transformMap) {
      const transformClass = this.transformMap[key]
      const tmp = this.moduleRef.get(transformClass).transform(val, metadata)
      result[key] = await Promise.resolve(tmp)
    }

    if (this.transformIdentity) {
      result['identity'] = val
    }

    return result
  }
}

/**
 * Take a map of property field name and transforms
 * Return a pipe that create the mapped values, kind of like combineReducer from redux
 * @param map
 * @param identity
 * @constructor
 */
export function MergePipe(
  map: { [key: string]: Type<PipeTransform> },
  identity: boolean = true
) {
  return mixin(
    class CustomMergePipe extends MergePipeTransform {
      protected readonly transformMap = map
      protected readonly transformIdentity = identity
    }
  )
}