Question

I have the following classes:

public abstract class Generic(){
    private int Id;
    ...
}

public class ExtA extends Generic(){
    private Generic fieldA();
    private Generic fieldB();
    ...
}

public class ExtB extends Generic(){
    private Generic fieldA;
    ....
}

public class ExtC extends Generic(){
    ...
}

Sorry for the vague example.

I am trying to find a way to save those objects to the database, but can’t seem to find a good way. I wish to have separate tables for ExtA, ExtB, ExtC, and then use foreign keys to relate the contained fields. I am using MySQL, and working with Java, Spring, Hibernate. Can someone please show me an example of how to do this, or point me to some tutorial?

I should mention I am new at working with databases.

Answer 1

Something like this. i’m relatively sure the Table_Per_Class strategy will let you ask for any “Generic” object and query all it’s subclass tables. Keep in mind if you do this, the ID should be unique across all of your subclass types. You’ll have to figure out a strategy for this use the right annotations to tell hibernate what to do. But in the mean time, this example assumes you’re assigning it manually before saving.

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Generic {
   @Id @Column(name="ID")
   private Long id;
   ...
}

@Entity
@Table(name="EXTA")
public class ExtA extends Generic {
   @Column(name="fieldA")
   private Generic fieldA;
}