I’m using class transformer to change the shape of my response in my typescript nestjs application. This is my initial structure:

[
  {
    "languages": [
      {
        "language_shortcode": "de",
        "label": "ASP-Einrichtung"
      },
      {
        "language_shortcode": "en",
        "label": "Some text"
      }
    ]
  },
  {
    "languages": [
      {
        "language_shortcode": "de",
        "label": "Deutschland"
      },
      {
        "language_shortcode": "en",
        "label": "USA"
      }
    ]
  }
]

I want to only return German flags, where language_shortcode is de.

This is what I did:

const serialized = plainToInstance(CategoryResponseDto, entries, {
  excludeExtraneousValues: true,
});

This is my dto:

export class CategoryResponseDto {
  @Expose()
  @Transform(
    ({ value }) => {
      const de_lang = value.find((lang) => lang.language_shortcode === 'de');
      return de_lang.label;
    },
    { toPlainOnly: true },
  )
  languages: Language[];

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

I get such a thing:

[
  {
    "languages": "ASP-Einrichtung"
  },
  {
    "languages": "Deutschland"
  }
]

I don’t want to repeat useless languages objects. How can I achieve that?