Hibernate JPA Sequence (non
Is it possible to use a DB sequence for some column that is not the identifier/is not part of a composite identifier ?
I'm using hibernate as jpa provider, and I have a table that has some columns that are generated values (using a sequence), although they are not part of the identifier.
What I want is to use a sequence to create a new value for an entity, where the column for the sequence is NOT (part of) the primary key:
@Entity
@Table(name = "MyTable")
public class MyEntity {
//...
@Id //... etc
public Long getId() {
return id;
}
//note NO @Id here! but this doesn't work...
@GeneratedValue(strategy = GenerationType.AUTO, generator = "myGen")
@SequenceGenerator(name = "myGen", sequenceName = "MY_SEQUENCE")
@Column(name = "SEQ_VAL", unique = false, nullable = false, insertable = true, updatable = true)
public Long getMySequencedValue(){
return myVal;
}
}
Then when I do this:
em.persist(new MyEntity());
the id will be generated, but the mySequenceVal property will be also generated by my JPA provider.
Just to make things clear: I want Hibernate to generate the value for the mySequencedValue property. I know Hibernate can handle database-generated values, but I don't want to use a trigger or any other thing other than Hibernate itself to generate the value for my property. If Hibernate can generate values for primary keys, why can't it generate for a simple property?
Looking for answers to this problem, I stumbled upon this link
It seems that Hibernate/JPA isn't able to automatically create a value for your non-id-properties. The @GeneratedValue annotation is only used in conjunction with @Id to create auto-numbers.
The @GeneratedValue annotation just tells Hibernate that the database is generating this value itself.
The solution (or work-around) suggested in that forum is to create a separate entity with a generated Id, something like this:
@Entity
public class GeneralSequenceNumber {
@Id
@GeneratedValue(...)
private Long number;
}
@Entity
public class MyEntity {
@Id ..
private Long id;
@OneToOne(...)
private GeneralSequnceNumber myVal;
}
I found that @Column(columnDefinition="serial") works perfect but only for PostgreSQL. For me this was perfect solution, because second entity is "ugly" option.
Hibernate definitely supports this. From the docs:
"Generated properties are properties which have their values generated by the database. Typically, Hibernate applications needed to refresh objects which contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. Essentially, whenever Hibernate issues an SQL INSERT or UPDATE for an entity which has defined generated properties, it immediately issues a select afterwards to retrieve the generated values."
For properties generated on insert only, your property mapping (.hbm.xml) would look like:
<property name="foo" generated="insert"/>
For properties generated on insert and update your property mapping (.hbm.xml) would look like:
<property name="foo" generated="always"/>
Unfortunately, I don't know JPA, so I don't know if this feature is exposed via JPA (I suspect possibly not)
Alternatively, you should be able to exclude the property from inserts and updates, and then "manually" call session.refresh( obj ); after you have inserted/updated it to load the generated value from the database.
This is how you would exclude the property from being used in insert and update statements:
<property name="foo" update="false" insert="false"/>
Again, I don't know if JPA exposes these Hibernate features, but Hibernate does support them.
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