Null values

Our current mapping strategy limits our means to specify data integrity constraints. It is no longer possible to disallow null values for properties belonging to derived classes. We might want to disallow null values in the bankName property. Hibernate will generate a corresponding database attribute ❶:

Java
package inherit.v2;
	      ...
	      @Entity @DiscriminatorValue(value = "Bank account")
	      public class BankAccount extends BillingDetails {
	      String bankName;
	      @Column(nullable=false) public String getBankName() {return bankName;} ...
Sql
CREATE TABLE BillingDetails (
	      id bigint(20) NOT NULL AUTO_INCREMENT PRIMARY KEY,
	      bankName varchar(255) NOT NULL, ❶
	      ...

Looks good? Unfortunately the attempt to save a bank account ❶ yields a runtime exception ❶:

Java
package inherit.v2;
	      ...
	      public class Persist {
	      ...
	      final CreditCard creditCard = new CreditCard("4412 8334 4512 9416", 1, "05/18/15");
	      session.save(creditCard);
              
	      final BankAccount bankAccount = new BankAccount("1107 2 31", "Lehman Brothers", "BARCGB22");
	      session.save(bankAccount) ❶; ...
Sql
...
	      Feb 19, 2013 10:28:00 AM org.hibernate.tool.hbm2ddl.SchemaUpdate execute
	      INFO: HHH000232: Schema update complete
	      Hibernate: 
	      insert 
	      into
              BillingDetails
              (created, number, cardType, expiration, dataType) 
	      values
              (?, ?, ?, ?, 'Credit card')
	      Feb 19, 2013 10:28:00 AM org.hibernate.engine.jdbc.spi.SqlExceptionHelper logExceptions
	      WARN: SQL Error: 1364, SQLState: HY000
	      Feb 19, 2013 10:28:00 AM org.hibernate.engine.jdbc.spi.SqlExceptionHelper logExceptions
	      ERROR: Field 'bankName' doesn't have a default value 
	      Exception in thread "main" org.hibernate.exception.GenericJDBCException: 
	      Field 'bankName' doesn't have a default value ❶
	      ...
	      at inherit.v2.Persist.main(Persist.java:28)
	      Caused by: java.sql.SQLException: Field 'bankName' doesn't have a default value

Conclusion: A table per class hierarchy mapping does not allow to specify not null constraints for properties of derived classes.

exercise No. 12

Mapping figures

Q:

Map the following model to a database:

Figure 1056. Figure subclasses
Figure subclasses

The two properties xCenter and yCenter in the abstract base class Figure represent the coordinates of the concrete figure's center of gravity. In a drawing application this would be considered the placement of the respective object.

The abstract method getArea() is meant to be implemented without interfering with your database mapping. Choose an integer discriminator. Test your application by storing and loading objects.

A:

The main difference to the current inherit.v1.BillingDetails example is the javax.persistence.Transient annotation of the area property in inherit.v3.Figure, inherit.v3.Circle and inherit.v3.Rectangle. The storage ant retrieval applications are inherit.v3.Persist, inherit.v3.RetrieveRectangles and inherit.v3.RetrieveAll are straightforward.