Inheritance with Java and Hibernate - Typecasting on updates

Question

I have the following hierarchy in my classes

@Entity
@Table(name = "Parent")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Parent {
}

@Entity
@DiscriminatorVlaue("FirstChild")
public class FirstChild extends Parent {
}

@Entity
@DiscriminatorVlaue("SecondChild")
public class SecondChild extends Parent {
}

This creates a table Parent as expected.

Some business logic in my app:

As the request is accepted , it is persisted as "Parent" type and creates the following entry in the table

Dtype    id  value
Parent   1   "some value"

As the request is processed it could either be of type FirstChild or of type SecondChild so in my code somewhere I have

if (some condition is met) {
// change the Parent to its firstChild type
}
else {
// chnage it to "SecondChild" type
}

Is my understanding and usage of inheritance correct? I am essentially downcasting objects in the if loop which throw runtime exceptions, but does changing the type of an object also change the DTYPE value in the database? I am essentially using inheritance to keep things organized as their types. Is there some magic I can do in the constructors to achieve this? But the real question is is the Dtype modifiable on an update?

To work with this further,

I created contsructors in the child classes as

public FirstChild(Parent parent) {
id = parent.id
}

but this creates a new row for the sub types ,

  Dtype       id  value
  Parent      1   "some value"
  FirstChild  2   "some value"

I am making sure that the "firstChild" record is created with the parent ID (1) but I am not sure why a second row is created

Answer 1

There is not a real inheritance in relational databases. Hibernate is just simulating it. Inheritance is a programming concept to use other classes like an owner. But you can't use other table as owning table.. In an child table if you have an element than you have a parent but where is the parent ? Parent is an element too.. ( a row of data ). So you simulate it in databases. Hibernate has some inheritance strategies.'SINGLE_TABLE' is one of them and collects parent table and child tables in 1 table. To differantiate which type is which class uses 'DiscriminatorColumn' on database. This column cant be changed it is static and declared with your java class. You can use 'DiscriminatorValue' to select a value to identify a java class at 'DiscriminatorColumn'. So your inheritance understanding is differs on relational databases. You must declare every child row data on parent table rows...

Answer 2

But the real question is is the Dtype modifiable on an update?

No, you cant: source.

From your example your pure java code would look like this:

Parent parent = new Parent(); //object received from request, stored in database as it is
if (some conditions is met) {
   // FirstChild firstChild = (Parent) parent; //this would throw ClassCastException
   FirstChild firstChild = new FirstChild(parent); //creating new object, gc will collect parent
}

So hibernate act same way as java: you cannot update Parent, you have to create new record for firstChild and delete parent.

Wouldn't be helpfull for you use @DiscriminatorFormula instead of checking some condition is met when processing the request?

@DiscriminatorFormula("case when value = 'some value' then 'FirstChild' else 'SecondChild' end")

Then you wouldnt have to save Parent and then process its records.