question: how to use exposing method inside a nested object array dto? about class-transformer

[andreasvh-conceto]

Thank you for the detailed explanation. This helps!
Best regards :)

[diffy0712]

Hi @andreasvh-conceto,
Thank you for the detailed issue and sorry for a late reply.

The error you got is a typescript error and nothing to do with this library.
The error clearly states that 'Types of property 'photos' are incompatible.' You used the Partial<AlbumDto>, but AlbumDto contains 'photos: PhotoDto[]', which is not Partial, so when you instantiate with an object containing photos, it will require you to be compatible with 'PhotoDto'. (so Partial is not a deep Partial).

But this is not the way you would want to use class-transformer. Here is a snippet for your scenario which will work:

class AlbumDto {
  id: number;
  name: string;

  @Type(() => PhotoDto)
  photos: PhotoDto[];

  @Expose()
  get title(): string {
    return "My super album";
  }
}

class PhotoDto {
  @Expose()
  filename: string;

  @Expose()
  get author(): string {
    return "John";
  }
}

describe("transmission detail response dto serialization test", () => {
  it("learning test nested objects", () => {
    const albumPartial = {
      id: 1,
      name: "My Album",
      photos: [{ filename: "photo1.jpg" }, { filename: "photo2.jpg" }],
    };

    const album = plainToInstance(AlbumDto, albumPartial);
    expect(album.id).toEqual(1);
    expect(album.name).toEqual('My Album');
    expect(album.photos[0].filename).toEqual('photo1.jpg');
    expect(album.photos[1].filename).toEqual('photo2.jpg');
    
    expect(instanceToPlain(album)).toEqual({
      id: 1,
      name: 'My Album',
      photos: [
        { filename: 'photo1.jpg', author: 'John' },
        { filename: 'photo2.jpg', author: 'John' }
      ],
      title: 'My super album'
    });
  });
});

[andreasvh-conceto]

Hi @diffy0712
one question came into my mind, while playing around with your answer. When i choose to @Expose({name: "myPropertyName"}) the plainToInstance method returns undefined for the annotated property. Currently i handle that using the ignoreDecorators: true option like this:

    const album = plainToInstance(AlbumDto, albumPartial, {
      ignoreDecorators: true, // this prevents that the property will be undefined, since it is keeping the property name from the created instance
    });

Is this the right approach to go for?

Thanks a lot and best regards

[diffy0712]

Hello @andreasvh-conceto,

if you provide @Expose({name: "myPropertyName"}) decorator to the name property, when calling plainToInstane, it will expect your plain to have a property called 'myPropertyName' instead of the real property name, which I assume you did not provide in your plain object, so it was set to undefined.

I would not suggest to use ignoreDecorators for this, as it is the same as not using the decorator in your situation.

So updating my previous example:

// if you add 

@Expose({name: "myPropertyName"})
name: string;

// then you plain would needs to change to 
const albumPartial = {
  id: 1,
  myPropertyName: "My Album",
  photos: [{ filename: "photo1.jpg" }, { filename: "photo2.jpg" }],
};

NOTE: if you override the name in expose, then you will get that name in your instanceToPlain result as well. If you need to override name only on one way please use the options toPlainOnly or toClassOnly. @Expose({name: "myPropertyName", toClassOnly: true})

Please read the documentation a bit more in detail, because everything I have wrote here is already in the documentations!

Hope this helps.