Question

I have a class mapped as an Entity to persist it in a database. I have an id field as the primary key so every time the object is persisted the value of the id is retrieved from the sequence “myClass_pk_seq”, a code like the following one.

@Entity
@Table(name="myObjects")
public class MyClass {
    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="sequence")
    @SequenceGenerator(name="sequence", sequenceName="myClass_pk_seq", allocationSize=1)
    @Column(name="myClassId")
    private Integer id;

    private Integer code;


    ...
}

I need to make in “code” attribute something similar to id. I need to have a sequence so I can assign to code the next value of the sequence (to reserve that value in case the user wants to reserve it but without persisting data). I mean the user will see the field, if he doesn’t know what to enter he can push a button and receieve in the screen the next possible value (and he can or not accept it). How can I get the next value of a sequence defined in JPA (and increment its value) without persisting the data for a non primary key field?

I just want to have a method which call nextval on a sequence associated with “code” field, and returns the value. What’s the best way to do it in JPA with annotations?

Thanks.

Answer 1

I just want to have a method which call nextval on a sequence associated with “code” field, and returns the value. What’s the best way to do it in JPA with annotations?

• Use native SQL to get the next sequence value when the user pushes the button. Either create the sequence manually or use a “fake entity” to have JPA create it for you.
• If you don’t want to use native SQL, insert an entity relying on the sequence and gets its id.

Both solutions sounds a bit ugly. Maybe you could simply use a random generator like a UUID generator.

Actually, you didn’t mention anything about the uniqueness of the code (and the JPA annotations don’t show it must be unique). Why don’t you return a random int?