How to serialize a nest js response with class-transformer while getting data with Typegoose?

Question

I have been trying to work through the NestJs example for the Serialization Section for Mongodb using Typegoose using the class-transformer library. The example given at https://docs.nestjs.com/techniques/serialization only shows how to use serialization in TypeORM. I followed the same process for Typegoose. Here is what I have tried so far.

// cat.domain.ts

import { prop } from '@typegoose/typegoose';

export class Cat {
  @prop()
  name: string;

  @prop()
  age: number;

  @prop()
  breed: string;
}


// cats.service.ts

@Injectable()
export class CatsService {
  constructor(
    @InjectModel(Cat) private readonly catModel: ReturnModelType<typeof Cat>,
  ) {}

  findAll(): Observable<Cat[]> {
    return from(this.catModel.find().exec());
  }

  findOne(id: string): Observable<Cat> {
    return from(this.catModel.findById(id).exec());
  }
  ...
}

// cat.response.ts

import { ObjectId } from 'mongodb';
import { Exclude, Transform } from 'class-transformer';

export class CatResponse {
  @Transform(value => value.toString(), { toPlainOnly: true })
  _id?: ObjectId;

  name: string;

  age: number;

  @Exclude()
  breed: string;

  constructor(partial: Partial<CatResponse>) {
    Object.assign(this, partial);
  }
}

// cats.controller.ts

@Controller('cats')
@UseInterceptors(ClassSerializerInterceptor)
export class CatsController {
  constructor(private readonly catsService: CatsService) {}

  @Get()
  findAll(): Observable<CatResponse[]> {
    return this.catsService.findAll();
  }

  @Get(':id')
  findOne(@Param() params: FindOneParamsDto): Observable<CatResponse> {
    return this.catsService.findOne(params.id);
  }
  ...
}

I tried running the API call on Get() with id but instead of the breed being excluded from the response I have been getting the following response.

{
    "$__": {
        "strictMode": true,
        "selected": {},
        "getters": {},
        "_id": {
            "_bsontype": "ObjectID",
            "id": {
                "type": "Buffer",
                "data": [
                    94,
                    93,
                    76,
                    66,
                    116,
                    204,
                    248,
                    112,
                    147,
                    216,
                    167,
                    205
                ]
            }
        },
        "wasPopulated": false,
        "activePaths": {
            "paths": {
                "_id": "init",
                "name": "init",
                "age": "init",
                "breed": "init",
                "__v": "init"
            },
            "states": {
                "ignore": {},
                "default": {},
                "init": {
                    "_id": true,
                    "name": true,
                    "age": true,
                    "breed": true,
                    "__v": true
                },
                "modify": {},
                "require": {}
            },
            "stateNames": [
                "require",
                "modify",
                "init",
                "default",
                "ignore"
            ]
        },
        "pathsToScopes": {},
        "cachedRequired": {},
        "$setCalled": [],
        "emitter": {
            "_events": {},
            "_eventsCount": 0,
            "_maxListeners": 0
        },
        "$options": {
            "skipId": true,
            "isNew": false,
            "willInit": true
        }
    },
    "isNew": false,
    "_doc": {
        "_id": {
            "_bsontype": "ObjectID",
            "id": {
                "type": "Buffer",
                "data": [
                    94,
                    93,
                    76,
                    66,
                    116,
                    204,
                    248,
                    112,
                    147,
                    216,
                    167,
                    205
                ]
            }
        },
        "name": "Sylver",
        "age": 14,
        "breed": "Persian Cat",
        "__v": 0
    },
    "$locals": {},
    "$op": null,
    "$init": true
}

Can anyone help me with how to serialize response properly?

Answer 1

UPDATE: class-transformer now works correctly with typegoose, look here for the documentation on how to use it

---

this is an known issue (#108), typegoose (& mongoose) are incompatible with class-transformer/class-validator
this is because typegoose needs to translate the class into an schema and mongoose will compile it to an model (which isnt the class anymore)