Using an Entity (and their Primary Key) as another Entity's Id

Question

So, I'm not sure how to ask this question, as it seems like it should be pretty easy to find the answer to this one.

I have 3 tables; ContentHeader, ContentType1 and ContentType2. ContentHeader has a primary, auto-increment key. ContentType1 and ContentType2 both maintain foreign keys to ContentHeader's primary key. These foreign keys are also the primary keys for their respective tables.

CREATE TABLE contentHeader (contentID INT AUTO_INCREMENT PRIMARY KEY, ...) ENGINE=InnoDB;

CREATE TABLE contentType1 (contentID INT PRIMARY KEY, FOREIGN KEY (contentID) REFERENCES contentHeader (contentID), ...) ENGINE=InnoDB;

CREATE TABLE contentType2 (contentID INT PRIMARY KEY, FOREIGN KEY (contentID) REFERENCES contentHeader (contentID), ...) ENGINE=InnoDB;

I have created four classes:

@Entity
public class ContentHeader {

  @Id
  @GeneratedValue
  protected int contentID;

  ...
}

@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public abstract class Content {

  @Id
  @OneToOne
  protected ContentHeader contentHeader;

  ...
}

@Entity
public class ContentType1 extends Content {
  ...
}

@Entity
public class ContentType2 extends Content {
  ...
}

This throws a null pointer when trying to generate the schema. I'm pretty sure I'm just missing something simple. I noticed the PrimaryKeyJoinColumn, but I'm not sure if it's what I need or not.

Answer 1

You can create a composite id class that only have the ContentHeader:

@Embeddable
public class ContentKey implements java.io.Serializable {

    @ManyToOne(cascade = {}, fetch = FetchType.LAZY)
    @JoinColumn(name = "ID", updatable = true)
    private ContentHeader header;
    // ...
}

@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public abstract class Content {

    @Id
    public ContentKey  getContentKeyId()
    // ...
}

That should do the trick.