My REST service has 3 layers:

1. Persistence Layer - database entities live here


2. Business Logic Layer - business objects live here


3. Access Layer - data transfer objects live here

The flow for reading a single entity (/entity/id) is:

Database Entity Object --> Business Object --> DTO

The trick is that the DTO should contain additional field compared to the Business Object or the Database Entity Object.

To be more exact, I have an entity User. The data model across layers is:

UserTable --> UserBO --> UserDTO

Every user has a profile photo, and the DTO should contain the URL of the profile photo, likewise:

 "firstName" : "David",
 "lastName" : "Beckham",
 "profilePhotoURL" : "amazon.s3.com/123"

Now the thing is that I don't save the URLs to the database, because they have an expiry date. Instead I just fetch them from Amazon S3 on the fly. This means that the UserTable and UserBO will not have the field profilePhotoURL, but UserDTO will have it.

The Question

What is the good way to convert the UserBO to UserDTO, but to still give it the additional field (profilePhotoURL)?

The URL is fetched from the service like String url = urlService.getUrlForUser(userId);

I was thinking to design my code to something similar to this:

public UserDTO getUser(long userId) 
 return dbService
 .fetchById(userId) // UserBO at this point
 .toDto() // UserDTO at this point, but missing URL
 .withUrl(urlService.getUrlForUser(userId)); // concat the URL to the DTO

The downside of this approach is that the client code can "forget" to call the withUrl(...) which will result with the DTO lacking the photo URL. Another downside would be if additional fields are required in time, then I would have more and more withX(), withY() methods, making the client code harder to maintain.

Any idea on how to make this design better?