Where to implement method which includes saving Embedded data? Repository or Service?

Question

I have collection like these:

class Book
{
  @Id String id;
  Publisher embeddedPublisher;
  ...
}

To create Book initially, I need to access on Publisher collection. For doing this, I think I have two choices:

Method 1. Implement CustomRepository method like using find or lookup

class BookCustomRepositoryImpl extends ... {
  MongoTemplate template;

  Book createBookWithPublisher(String bookName, String publisherName) {
    // With MongoTemplate, find Publisher by publisherName
    // With MongoTemplate, save Book with retrieved Publisher data
  }
}

Method 2. Implement logic on @Service

@Service
class BookService {
  BookRepository bookRepo;
  PublisherRepository publisherRepo;

  BookDto createBookWithPublisher(String bookName, String publisherName) {
    // With PublisherRepo, find Publisher by publisherName
    // With BookRepo, save Book with retrieved Publisher data
  }
}

What would be better practice regarding logic separation? Share your opinions!

Thank you!