Invalid behaviors in Multipart-form transform and validation

Bug Report

I encountered a couple of validation and transformation issues using multipart-forms. I collected them in the following points and an isolated sample project.

Current behavior

1. if the content type is multipart-form then transform for the nested object doesn’t work. (nested property postalAddress must be either object or array).
2. As the multipart-form conversion doesn’t happen automatically. I manually add the transform (CreateUser.dto.ts line 35), but then the validation doesn’t work in the nested object (I have tried to post an empty field that was required and it passed the validation)
3. If I force the Type for a number field (CreateUser.dto.ts line 28) then the empty value will be transformed to “0” that prevents from validate after if the parameter was empty or not.
4. I can’t validate the file in the DTO if it was empty, because the file is not part of the @Body.

Input Code

https://github.com/andras-danko-dh/nest-swagger-multipart-form

export class CreateUser {

  @ApiProperty({ 
    description: 'The name of the user',
    example: 'Who Knows'
  })
  @IsString()
  @IsNotEmpty() //TODO: Why it is needed if its a required field?
  readonly name: string;

  @ApiProperty({ 
    description: 'The email address of the owner',
    example: '[e-mail]'
  })
  @IsEmail()
  readonly email: string;

  @ApiPropertyOptional({ 
    description: 'Age of the user',
    example: 25
  })
  // @Type(() => Number)  //TODO: Why should we do manually the type conversion?
  @IsInt()
  readonly age: number;

  @ApiProperty({ 
    description: 'Postal address'
  })
  // @Transform(value => JSON.parse(value)) //If we add it the transform works, but the validation didn't
  @Type(() => Address) //TODO: Type conversion didn't work if we have multipart-form
  @ValidateNested()
  @IsNotEmptyObject() //TODO: Why it is needed if its a required field
  readonly postalAddress: Address

  @ApiProperty({ 
    description: 'Avatar image of the user',    
    type: 'string', 
    format: 'binary' 
  })
  @IsNotEmpty()
  readonly avatar: any;  

};

Expected behavior

1. If in the Swagger descriptor a field type is an object then it should be converted to a json object automatically and validate based on the object type apart from the content type.
2. The validation should also work on nested objects in case it was converted (maybe the first point resolved this too?)
3. @Type conversion should keep the value as NaN/Null/Undefined if the field had no value
4. Be able to validate that the file is not empty

Possible Solution

Environment

Nest version: 6.14.1
 
For Tooling issues:
- Node version: 12
- Platform:  Linux

Others:

[zerox12311]

Dear @kamilmysliwiec, has any document to explain what to validate the file is not empty?

[SerdarN92]

I have the same problem. Any solution? @kamilmysliwiec