Hibernate中的Entity類之間的繼承關系之三JOINED

對于Hibernate提供的4種相容JPA的映射政策，這裡介紹第三種Joined table，也被稱為table-per-subclass政策。 

在這種政策中，存在如下特征：

• 父子Entity類都對應各自的資料庫表
• 父表中無需設定discriminator列
• 子表中的主鍵ID，也是子表的外鍵，用以指向父表的主鍵ID。預設父子表中的主鍵名字相同，也可以在子Entity類中通過如下标注給出列名。

  @javax.persistence.PrimaryKeyJoinColumn(name="CUST_ID")
        

• 通路子Entity類時需要在資料庫中join通路父Entity類

父Entity類定義如下：

@Entity(name = "Account")
@javax.persistence.Inheritance(strategy = InheritanceType.JOINED)
public static class Account {

    @Id
    private Long id;

    private String owner;

    private BigDecimal balance;

    private BigDecimal interestRate;

    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }

    public String getOwner() {
        return owner;
    }
    public void setOwner(String owner) {
        this.owner = owner;
    }

    public BigDecimal getBalance() {
        return balance;
    }
    public void setBalance(BigDecimal balance) {
        this.balance = balance;
    }

    public BigDecimal getInterestRate() {
        return interestRate;
    }
    public void setInterestRate(BigDecimal interestRate) {
        this.interestRate = interestRate;
    }
}
           

子 Entity類定義如下：

@Entity(name = "DebitAccount")
@javax.persistence.PrimaryKeyJoinColumn(name = "account_id")
public static class DebitAccount extends Account {
    private BigDecimal overdraftFee;

    public BigDecimal getOverdraftFee() {
        return overdraftFee;
    }
    public void setOverdraftFee(BigDecimal overdraftFee) {
        this.overdraftFee = overdraftFee;
    }
}
           

另一個子Entity類定義如下：

@Entity(name = "CreditAccount")
@javax.persistence.PrimaryKeyJoinColumn(name = "account_id")
public static class CreditAccount extends Account {

    private BigDecimal creditLimit;

    public BigDecimal getCreditLimit() {
        return creditLimit;
    }

    public void setCreditLimit(BigDecimal creditLimit) {
        this.creditLimit = creditLimit;
    }
}
           

資料庫表結構如下：

CREATE TABLE Account (
    id BIGINT NOT NULL ,
    balance NUMERIC(19, 2) ,
    interestRate NUMERIC(19, 2) ,
    owner VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE CreditAccount (
    creditLimit NUMERIC(19, 2) ,
    account_id BIGINT NOT NULL REFERENCES Account(id),
    PRIMARY KEY ( id )
)

CREATE TABLE DebitAccount (
    overdraftFee NUMERIC(19, 2) ,
    account_id BIGINT NOT NULL REFERENCES Account(id),
    PRIMARY KEY ( id )
)