InjectRepository with generic type T

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[ ] Bug report
[x] Feature request
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I would like to be able to inject a Repository with a generic type. Do you think it is possible?

Current behavior

@Injectable()
export class ResourcesService<T> {
  constructor(
    @InjectRepository(T)                                // <-- error happens here
    private readonly repo: Repository<T>
  ) {}
}

Typescript error:

error TS2693: 'T' only refers to a type, but is being used as a value here.

This is the only way I can think of doing it, and it is not a valid way.

[krazibit]

Finally managed to do it, below is a code snippet you might find useful

import {Type} from '@nestjs/common'
import {InjectRepository} from '@nestjs/typeorm'
import * as DataLoader from 'dataloader'
import {keyBy} from 'lodash'
import {Repository} from 'typeorm'

export interface IDataService<T> {
  readonly repository: Repository<T>
  load: (id: string | number) => Promise<T>
  loadMany: (ids: Array<string | number>) => Promise<T[]>
}

type Constructor<I> = new (...args: any[]) => I // Main Point

export function DataService<T>(entity: Constructor<T>): Type<IDataService<T>> {
  class DataServiceHost implements IDataService<T> {
    @InjectRepository(entity) public readonly repository: Repository<T>

    private get primaryColumnName(): string {
      return this.repository.metadata.primaryColumns[0]?.propertyName
    }

    private loader: DataLoader<number | string, T> = new DataLoader(async ids => {
      const entities = await this.repository.findByIds(ids as any[])
      const entitiesKeyed = keyBy(entities, this.primaryColumnName)
      return ids.map(id => entitiesKeyed[id])
    })

    public async load(id: string | number): Promise<T> {
      return this.loader.load(id)
    }

    public async loadMany(ids: Array<string | number>): Promise<T[]> {
      return this.loadMany(ids)
    }
  }
  return DataServiceHost
}

Not you can extend you services with this as explained by @BrunnerLivio here https://github.com/nestjs/typeorm/issues/187#issuecomment-528910193

[incompletude]

Finally managed to do it, below is a code snippet you might find useful

import {Type} from '@nestjs/common'
import {InjectRepository} from '@nestjs/typeorm'
import * as DataLoader from 'dataloader'
import {keyBy} from 'lodash'
import {Repository} from 'typeorm'

export interface IDataService<T> {
  readonly repository: Repository<T>
  load: (id: string | number) => Promise<T>
  loadMany: (ids: Array<string | number>) => Promise<T[]>
}

type Constructor<I> = new (...args: any[]) => I // Main Point

export function DataService<T>(entity: Constructor<T>): Type<IDataService<T>> {
  class DataServiceHost implements IDataService<T> {
    @InjectRepository(entity) public readonly repository: Repository<T>

    private get primaryColumnName(): string {
      return this.repository.metadata.primaryColumns[0]?.propertyName
    }

    private loader: DataLoader<number | string, T> = new DataLoader(async ids => {
      const entities = await this.repository.findByIds(ids as any[])
      const entitiesKeyed = keyBy(entities, this.primaryColumnName)
      return ids.map(id => entitiesKeyed[id])
    })

    public async load(id: string | number): Promise<T> {
      return this.loader.load(id)
    }

    public async loadMany(ids: Array<string | number>): Promise<T[]> {
      return this.loadMany(ids)
    }
  }
  return DataServiceHost
}

Not you can extend you services with this as explained by @BrunnerLivio here #187 (comment)

This is very nice. Can avoid a lot of boilerplate code.