Enterprise Java Development@TOPIC@

Chapter 64. Inheritance Strategy:Table per Concrete Class

64.1. Table per Concrete Class Strategy Overview
64.2. Table per Concrete Class Example Database Schema
64.3. Table per Concrete Class Example Java Mapping
64.4. Table per Concrete Class Example Usage
64.5. Summary

64.1. Table per Concrete Class Strategy Overview

Figure 64.1. Table per Concrete Class Inheritance Strategy

Table per Concrete Class Inheritance Strategy

  • Advantages

    • May have constrained columns

    • No joins when accessing a single concrete type

  • Disadvantages

    • Not normalized

    • Redundant columns in each concrete child table

    • More work required to query across tables

      • Requires use of SQL "UNION"

    • Least desirable from a performance/portability standpoint

  • More suitable for ...

    • Sub-types not needed to be manipulated with sibling other sub-types

64.2. Table per Concrete Class Example Database Schema

Figure 64.2. Table per Concrete Class Example Database Schema

create table ORMINH_CHECKING (
    id bigint not null,
    balance double not null,
    fee double not null,
    primary key (id)
)
create table ORMINH_INTERESTACCT (
    id bigint not null,
    balance double not null,
    rate double not null,
    primary key (id)
)
create sequence ORMINH_SEQ

  • Table for each concrete class

  • No separate table for parent class

  • Parent columns repeated in concrete sub-class tables

  • * his particular example uses SEQUENCE for primary key generation

64.3. Table per Concrete Class Example Java Mapping

Figure 64.3. Table per Concrete Class Example Java Mapping (Parent Class)

@Entity

@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)

@SequenceGenerator(

        name="orminhSeq", //required logical name

        sequenceName="ORMINH_SEQ" //name in database

)

public abstract class Account {

    @Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="orminhSeq")

    private long id;

    private double balance;

...

    public void deposit(double amount) throws AccountException {

        setBalance(getBalance() + amount);

    }

    public abstract void withdraw(double amount) throws AccountException;

...

  • Parent defines default mapping for all derived types, including PK generation

  • Using a common SEQUENCE allows sub-classes to have unique PKs across all tables

Figure 64.4. Table per Concrete Class Example Java Mapping (Subclasses)

@Entity

@Table(name="ORMINH_CHECKING")

public class CheckingAccount extends Account {

    private double fee;


    public void withdraw(double amount) throws AccountException {

        super.setBalance(super.getBalance() - fee);

    }
@Entity

@Table(name="ORMINH_INTERESTACCT")

public class InterestAccount extends Account {

    private double rate;

    

    public void withdraw(double amount) throws AccountException {

        super.setBalance(super.getBalance() - amount);

    }

  • Subclasses name their specific entity class table

64.4. Table per Concrete Class Example Usage

Figure 64.5. Table per Concrete Class Example Usage (Persist)

ejava.examples.orm.inheritance.annotated.CheckingAccount checking = new CheckingAccount();

checking.setFee(0.50);

em.persist(checking);


ejava.examples.orm.inheritance.annotated.InterestAccount savings = new InterestAccount();

savings.setRate(0.25);

em.persist(savings);

Hibernate:
    call next value for ORMINH_SEQ
Hibernate:
    insert into ORMINH_CHECKING (balance, fee, id) 
    values (?, ?, ?)
Hibernate:
    insert into ORMINH_INTERESTACCT (balance, rate, id) 
    values (?, ?, ?)

  • Rows for entities placed into separate tables

Figure 64.6. Table per Concrete Class Example Usage (Get Entities)

List<Account> accounts =em.createQuery("select a from Account a").getResultList(); 

assertTrue("unexpected number of accounts:" + accounts.size(), accounts.size() == 2);

for(Account a: accounts) {

    log.info("account found:" + a);

}
select
    account0_.id as id1_0_,
    account0_.balance as balance2_0_,
    account0_.rate as rate1_7_,
    account0_.fee as fee1_2_,
    account0_.clazz_ as clazz_ 
from ( 
    select id, balance, rate, null as fee, 1 as clazz_ 
    from ORMINH_INTERESTACCT
     
    union all
     
    select id, balance, null as rate, fee, 2 as clazz_ 
    from ORMINH_CHECKING 
) account0_
-account found:InterestAccount, id=51, balance=0.0, rate=0.25
-account found:CheckingAccount, id=50, balance=0.0, fee=0.5

  • Query through parent type causes SQL "UNION ALL" of each concrete sub-class table

Figure 64.7. Table per Concrete Class Example Usage (Verify DB Schema)

//query specific tables for columns

int rows = em.createNativeQuery(

        "select ID, BALANCE, FEE from ORMINH_CHECKING")

        .getResultList().size();

assertEquals("unexpected number of checking rows:" + rows, 1, rows);

rows = em.createNativeQuery(

        "select ID, BALANCE, RATE from ORMINH_INTERESTACCT")

        .getResultList().size();

assertEquals("unexpected number of interestacct rows:" + rows, 1, rows);
select * from ORMINH_CHECKING
ID BALANCE FEE
-- ------- ---
50 0.0     0.5

select * from ORMINH_INTERESTACCT
ID BALANCE RATE
-- ------- ----
51 0.0     0.25

  • Table per concrete class

  • No unused columns

  • Parent columns repeated in each sub-class table

64.5. Summary