uniqueConstraintValidation.ts

async renameFolder (folderId: number, name: string): Promise<Folder> {
  // this is an additional DB call in case we want to do app validation
  // could be avioded if we run update opration directly
  // in most cases it's impact is negligible (but depends on our overall usecase DB throughput)
  const folder = await this.dbRepo.findOneBy({
    id: folderId
  })
  
  // app layer validation - it's going to work in most of the cases
  if (folder.name === name) {
    throw new ConflictException('Folder with such name already exists')
  }

  folder.name = name

  try {
    // we still have to try catch this opperation since very rarelly but it might happen that the check above is not enough
    // e.g. due to another rename operation being executed at the same time
    await this.folderRepository.save(folder)
  } catch (error) {
     // here we try to cast all known / expected DB exectpions to app one
     // e.g. unique constraints or other things
     if (error instanceof TypeORMError && <is duplicate error>) {
        throw new ConflictException('Folder with such name already exists')
     }
     
     logger.error('Failed to reaname folder', {
        folderId,
        name,
        error
     })
     
     // in case the operation failed for an unknown reason we log the original error
     // but cast the DB error to an app one and bubble it up 
     throw new InternalServerErrorException('Error renaming folder')
  }

  retrun folder
}