Hello,
I created this classes:

@Entity
@Table(name="Part")
@IdClass(Part.class)
public class Part extends GenericPartSpecification {
@Id
@Column(name="SerialNumber", unique=true, nullable=false)
private String serialNumber;
...
}

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name="GenericPartSpecification")
public class GenericPartSpecification implements Comparable<GenericPartSpecification> {

@Id
@Column(name="PartNumber")
protected String partNumber;
}

The class Part inherits the primary key PartNumber from GenericPartSpecification. I want that the class Part has an additional primary key SerialNumber. The code above returns this Exception:
javax.persistence.PersistenceException: org.hibernate.AnnotationException: Unable to define/override @Id(s) on a subclass: de.fhg.atl.javabeans.Part

Is there a solution for that?

Alex

AFAIK it is not possible to have more than one primary key. It just doesn't make any sense.

Maybe what you want to use is a composite-id (http://www.hibernate.org/hib_docs/core/ ... ompositeid)

Stefon is correct, you can have only one primary key on a table (at least on all relational databases I've ever worked with). I believe what you want is a unique key and I see you have added the "unique = true" to the @Column annotation for the serial number field. Does that not enforce a unique serial number for every row in the table?

Thank you for your replies.
This is what I want to do:

CREATE TABLE GenericPartSpecification (
PartNumber VARCHAR(50) PRIMARY KEY,
Description VARCHAR(128),...)

CREATE TABLE Part (
SerialNumber VARCHAR(50) NOT NULL,
PartNumber VARCHAR(50) NOT NULL,
CreationDate DATETIME NOT NULL,
PRIMARY KEY (SerialNumber, PartNumber),
FOREIGN KEY (PartNumber) REFERENCES GenericPartSpecification (PartNumber) ON DELETE CASCADE ON UPDATE CASCADE)

I thought that would be the best to do that with inheritance. But the problem is to get a additional primary key in Part.