Creating a composite Unique constraints on multiple columns

Question

This is my model:

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

Every book number starts at 1 per author and increments upwards.

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; 
}