7장. 상속 매핑

요약

7.1 암시적 다형성을 활용한 구체 클래스별 테이블 - @MappedSuperclass

• 상위 클래스의 인스턴스 쿼리 시, 쿼리 n회(하위 클래스 개수만큼) 발생

  select bd from BillingDetails bd
  ---------
  SELECT ID, OWNER, ACCOUNT, BANKNAME, SWIFT
  FROM BANKACCOUNT;
  
  SELECT ID, OWNER, CARDNUMBER, EXPMONTH, EXPYEAR
  FROM CREDITCARD;
  

• 다형적 연관관계를 처리할 수 없다.

  • 예를들면, User에서 BillingDetails로의 연관관계가 불가하다.

7.2 유니온을 활용한 구체 클래스별 테이블 - @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)

• 테이블 구조는 7.1 @MappedSuperclass 와 동일함.

• 상위 클래스의 인스턴스 쿼리 시, union all 처리되어 쿼리 1회 발생

  select bd from BillingDetails bd
  ---------
  SELECT ID, OWNER, 
  	     ACCOUNT, BANKNAME, SWIFT,
  	     CARDNUMBER, EXPMONTH, EXPYEAR
  FROM
  (
  	SELECT ID, OWNER, 
  	       ACCOUNT, BANKNAME, SWIFT,
  				 null as CARDNUMBER, null as EXPMONTH, null as EXPYEAR
  	FROM BANKACCOUNT
  	
  	UNION ALL
  	
  	SELECT ID, OWNER, 
  	       null as ACCOUNT, null as BANKNAME, null as SWIFT,
  				 CARDNUMBER, EXPMONTH, EXPYEAR
  	FROM CREDITCARD
  ) as BILLINGDETAILS
  

• 다형적 연관관계를 처리할 수 있다.

  • 예를들면, User에서 BillingDetails로의 연관관계가 가능해진다.

7.3 클래스 계층 구조별 테이블 - @Inheritance(strategy = InheritanceType.SINGLE_TABLE)

select bd from BillingDetails bd
---------
SELECT ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER,
	     ACCOUNT, BANKNAME, SWIFT, BD_TYPE
  FROM BILLINGDETAILS

select cc from CreditCard cc
---------
SELECT ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER
  FROM BILLINGDETAILS
 WHERE BD_TYPE = 'CC'

7.4 조인을 활용한 하위 클래스별 테이블 - @Inheritance(strategy = InheritanceType.JOINED), @PrimaryKeyJoinColumn, @DiscriminatorColumn

select bd from BillingDetails bd
---------
SELECT BD.ID, BD.OWNER, 
			 CC.EXPMONTH, CC.EXPYEAR, CC.CARDNUMBER,
	     BA.ACCOUNT, BA.BANKNAME, BA.SWIFT, 
	     CASE
		     WHEN CC.CREDITCARD_ID is not null then 1
 		     WHEN BA.ID is not null then 2
 		     WHEN BD.ID is not null then 0
	     END
  FROM BILLINGDETAILS BD
  LEFT OUTER JOIN CREDITCARD CC ON CC.CREDIT_CARD = BD.ID
  LEFT OUTER JOIN BANKACCOUNT BA ON BA.ID = BD.ID

select cc from CreditCard cc
---------
SELECT CREDITCARD_ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER
  FROM CREDITCARD
 INNER JOIN BILLINGDETAILS BD ON BD.ID = CREDITCARD_ID

7.5 상속 전략 혼합 - @Inheritance(strategy = InheritanceType.SINGLE_TABLE), @DiscriminatorColumn, @SecondaryTable

SELECT ID, OWNER, ACCOUNT, BANKNAME, SWIFT,
        EXPMONTH, EXPYEAR, CARDNUMBER,
	      BD_TYPE
  FROM BILLINGDETAILS
  LEFT OUTER JOIN CREDITCARD on ID = CREDIT_CARD_ID

select cc from CreditCard cc
---------
SELECT CREDITCARD_ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER
  FROM CREDITCARD
 INNER JOIN BILLINGDETAILS BD ON BD.ID = CREDITCARD_ID

7.1 암시적 다형성을 활용한 구체 클래스별 테이블 - @MappedSuperclass

@MappedSuperclass
public abstract class BillingDetails { // 추상 클래스
	@Id
	@GeneratedValue(generator = "ID_GENERATOR")
	private Long id;

	@NotNull
	private String owner;
	
	// ...
}

@Entity
@AttributeOverride(
	name = "owner",
	column = @Column(name = "CC_OMNER", nulLable = false)
)
public class CreditCard extends BillingDetails {
	@NotNull
	private String cardNumber;

	@NotNull
	private String expMonth;

	@NotNull
	private String expYear;
	
	//...
}

@Entity
public class BankAccount extends BillingDetails {
	@NotNull
	private String account;
	
	@NotNull
	private String bankname;
	
	@NotNull
	private String swift;

	//...
}

@NoRepositoryBean
public interface BillingDetailsRepository<T extends BillingDetails, ID>
	extends JpaRepository<T, ID> {
	
	List<T> findByOwner(String owner);
}

public interface BankaccountRepository
		extends BillingDetailsRepository<BankAccount, Long> {
		
		List<BankAccount> findBySwift(String swift);
}

public interface CreditCardRepository
		extends BillingDetailsRepository<CreditCard, Long> {
		
		List<CreditCard> findByExpYear(String expYear);
}

• 단점

  • 상위 클래스에 대한 쿼리를 구체적인 각 하위 클래스에 대해 하나씩 여러 개의 SQL SELECT를 실행한다.

    select bd from BillingDetails bd
    ---------
    SELECT ID, OWNER, ACCOUNT, BANKNAME, SWIFT
    FROM BANKACCOUNT;
    
    SELECT ID, OWNER, CARDNUMBER, EXPMONTH, EXPYEAR
    FROM CREDITCARD;
    

• 다형적 연관관계를 처리할 수 없다.

  • 예를들면, User에서 BillingDetails로의 연관관계가 불가하다.

7.2 유니온을 활용한 구체 클래스별 테이블 - @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Billingbetails { // abstract
	@Id
	@GeneratedValue(generator = "ID_GENERATOR")
	private Long id;
	
	@NotNull
	private String owner;
	
	//...
}

@Entity 
@Attributeoverride(
	name = "owner",
	column = @Column(name = "CC_OWNER", nuLlable = false)
)
public class CreditCard extends BillingDetails {
	@NotNull
	private String cardNumber;
	
	@NotNull
	private String expMonth;
	
	@NotNull
	private String expYear;
	
	//...
}

@Entity
public class BankAccount extends BillingDetails {
	@NotNull
	private String account;
	
	@NotNull
	private String bankname;
	
	@NotNull
	private String swift;

	//...
}

public interface BillingDetailsRepository<T extends BillingDetails, ID>
	extends JpaRepository<T, ID> {
	
	List<T> findByOwner(String owner);
}

select bd from BillingDetails bd
---------
SELECT ID, OWNER, 
	     ACCOUNT, BANKNAME, SWIFT,
	     CARDNUMBER, EXPMONTH, EXPYEAR
FROM
(
	SELECT ID, OWNER, 
	       ACCOUNT, BANKNAME, SWIFT,
				 null as CARDNUMBER, null as EXPMONTH, null as EXPYEAR
	FROM BANKACCOUNT
	
	UNION ALL
	
	SELECT ID, OWNER, 
	       null as ACCOUNT, null as BANKNAME, null as SWIFT,
				 CARDNUMBER, EXPMONTH, EXPYEAR
	FROM CREDITCARD
) as BILLINGDETAILS

• 장점
  • 다형적 연관관계를 처리할 수 있다.
    • 예를들면, User에서 BillingDetails로의 연관관계가 가능해진다.

7.3 클래스 계층 구조별 테이블 - @Inheritance(strategy = InheritanceType.SINGLE_TABLE)

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "BD_TYPE")
public abstract class BillingDetails {
	@Id
	@GeneratedVa lue(generator = "ID_GENERATOR")
	private Long id;
	
	@NotNull
	@Column(nulLable = false)
	private String owner;
	
	// ...
}

@Entity 
@DiscriminatorValue("CC")
public class CreditCard extends BillingDetails {
	@NotNull
	private String cardNumber;
	
	@NotNull
	private String expMonth;
	
	@NotNull
	private String expYear;
	
	//...
}

select bd from BillingDetails bd
---------
SELECT ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER,
	     ACCOUNT, BANKNAME, SWIFT, BD_TYPE
  FROM BILLINGDETAILS

select cc from CreditCard cc
---------
SELECT ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER
  FROM BILLINGDETAILS
 WHERE BD_TYPE = 'CC'

레거시 스키마여서 엔티티 테이블에 판별자 칼럼을 추가로 포함할 수 없는 경우 - @DiscriminatorFormula

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@org.hibernate.annotations.DiscriminatorFormula(
	"case when CARDNUMBER is not nult then 'CC' else 'BA' end"
)
public abstract class BillingDetails {
}

7.4 조인을 활용한 하위 클래스별 테이블 - @Inheritance(strategy = InheritanceType.JOINED), @PrimaryKeyJoinColumn, @DiscriminatorColumn

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class BillingDetails {
	@Id
	@GeneratedVa lue(generator = "ID_GENERATOR")
	private Long id;
	
	@NotNull
	@Column(nulLable = false)
	private String owner;
	
	// ...
}

@Entity
// @PrimaryKeyJoinColumn(name = "BANKACCOUNT_ID") // 식별자를 상위 클래스로부터 상속
public class BankAccount extends BillingDetails {
	@NotNull
	private String account;
	
	@NotNull
	private String bankname;
	
	@NotNull
	private String swift;

	//...
}

@Entity 
@PrimaryKeyJoinColumn(name = "CREDITCARD_ID") // 식별자를 명시적으로 지정
public class CreditCard extends BillingDetails {
	@NotNull
	private String cardNumber;
	
	@NotNull
	private String expMonth;
	
	@NotNull
	private String expYear;
	
	//...
}

select bd from BillingDetails bd
---------
SELECT BD.ID, BD.OWNER, 
			 CC.EXPMONTH, CC.EXPYEAR, CC.CARDNUMBER,
	     BA.ACCOUNT, BA.BANKNAME, BA.SWIFT, 
	     CASE
		     WHEN CC.CREDITCARD_ID is not null then 1
 		     WHEN BA.ID is not null then 2
 		     WHEN BD.ID is not null then 0
	     END
  FROM BILLINGDETAILS BD
  LEFT OUTER JOIN CREDITCARD CC ON CC.CREDIT_CARD = BD.ID
  LEFT OUTER JOIN BANKACCOUNT BA ON BA.ID = BD.ID

select cc from CreditCard cc
---------
SELECT CREDITCARD_ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER
  FROM CREDITCARD
 INNER JOIN BILLINGDETAILS ON BD.ID = CREDITCARD_ID

• @DiscriminatorColumn 사용 가능
• 판별자 매핑을 무시하려면 hibernate.discriminator.ignore_explicit_for_joined를 활성화 하면 된다.

7.5 상속 전략 혼합 - @Inheritance(strategy = InheritanceType.SINGLE_TABLE), @DiscriminatorColumn, @SecondaryTable

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "BD_TYPE")
public abstract class BillingDetails {
	@Id
	@GeneratedVa lue(generator = "ID_GENERATOR")
	private Long id;
	
	@NotNull
	@Column(nulLable = false)
	private String owner;
	
	// ...
}

@Entity 
@DiscriminatorValue("CC")
@SecondaryTable(
	name = "CREDITCARD"
	pkJoinColumns = @PrimaryKeyJoinColumn(name = "CREDITCARD_ID")
)
public class CreditCard extends BillingDetails {
	@NotNull
	private String cardNumber;
	
	@NotNull
	private String expMonth;
	
	@NotNull
	private String expYear;
	
	//...
}

SELECT ID, OWNER, ACCOUNT, BANKNAME, SWIFT,
        EXPMONTH, EXPYEAR, CARDNUMBER,
	      BD_TYPE
  FROM BILLINGDETAILS
  LEFT OUTER JOIN CREDITCARD on ID = CREDIT_CARD_ID

7.6 임베드 가능한 클래스의 상속

• 임베드 가능한 클래스는 그것을 소유한 엔티티의 컴포넌트이므로, 일반적인 엔티티 상속 규칙은 적용되지 않는다.
• 하이버네이트 확장 기능으로 상위 클래스(또는 인터페이스)로부터 일부 영속성 프로퍼티를 상속하는 임베드 가능한 클래스를 매핑할 수 있다.

@MappedSuperclass
public abstract class Measurement {
	@NotNull
	private String name;
	
	@NotNull
	private String symbol;
	
	// ...
}

@Embeddable
@AttributeOverride(name = "name", column = @Column(name = "DIMENSIONS_NAME"))
@AttributeOverride(name = "symbol", column = @Column(name = "DIMENSIONS_SYMBOL"))
public class Dimensions extends Measurement {
	@NotNull
	private BigDecimal depth;
	
	@NotNull
	private BigDecimal height;
	
	@NotNull
	private BigDecimal width;
	
	//...
}

@Embeddable
@AttributeOverride(name = "name", column = @Column(name = "WEIGHT_NAME"))
@AttributeOverride(name = "symbol", column = @Column(name = "WEIGHT_SYMBOL"))
public class Weight extends Measurement {
	@NotNull
	@Column(name = "WEIGHT")
	private BigDecimal value;

	// ...
}

@Entity
public class Item {
	
	private Dimensions dimensions;
	
	private Weight weight;

	// ...
}

• 주의해야할 함정

7.7 전략 선택

• 어떤 상속 매핑 전략을 선택할지는 엔티티 계층 구조에서 상위 클래스가 어떻게 사용되느냐에 따라 달라진다.
• 또 다른 중요한 측면은 상위 타입과 하위 타입의 속성, 즉 하위 타입에 추가 속성이 많은지, 아니면 상위 타입과 동작만 다른지 여부