Question

This is my model:

class User {...}
class Book {
  User author;
  int number;
}

Every book number starts at 1 per author and increments upwards. So we’ll have Books 1,2,3 by John Grisham, Book 1..5 by George Martin, etc…

Is there a unique constraint I can place on Book, that would guarantee we don’t have two books with the same number by the same author? Similar to @Column(unique = true), but the constraint only applies on the composite of Author X number?

Answer 1

Use @UniqueConstraint:

@Table(
    uniqueConstraints=
        @UniqueConstraint(columnNames={"author_id", "number"})
)
@Entity
class Book extends Model {
   @ManyToOne
   @JoinColumn(name = "author_id")
   User author;
   int number; 
}