Question

I would like to generate a custom ID for an object, depending on values already in database.

I know several questions were asked on that subject, but I can’t figure a solution out…

Here is my class :

@Entity
class A {
    // primary key for table
    @GeneratedValue
    @Id
    private long tableId;

    // id -> should be generated as (1+ (max id of type 'type'))
    @Formula("1+(select t.id from mytable t where t.type=type)") 
    private long id;

    // type 
    private String type;
}         

I thought of the @Formula annotation, but I can’t get it work…

exception raised :

java.sql.SQLException: Field 'id' doesn't have a default value 

I’m not sure the @Formula is the good solution…

Does anybody has a clue of how I can make it work ?

Thanks a lot,

Ben

Answer 1

I solved my problem using a @PrePersist annotation.

@PrePersist
private void generateId() {

    if(id>0){
        return;
    }
    id=++type.lastChronoId;
    type.save();
}

with type modified to a Type class, containing the index of the last created object.