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空值和非空值 @DiscriminatorValue 选项

发布者：    |       讨论 Hibernate ORM

继承和辨别器列

尽管它可以用于 JOINED 表继承，但 @DiscriminatorValue 更常用于 SINGLE_TABLE 继承。对于 SINGLE_TABLE，辨别器列告诉 Hibernate 与每个特定的数据库行相关联的子类实体类型。

如果没有指定辨别器列，Hibernate 将使用默认的 DTYPE 列。为了可视化其工作原理，考虑以下领域模型继承层次结构

@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
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;
    }
}

@Entity(name = "DebitAccount")
public static class DebitAccount extends Account {

    private BigDecimal overdraftFee;

    public BigDecimal getOverdraftFee() {
        return overdraftFee;
    }

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

@Entity(name = "CreditAccount")
public static class CreditAccount extends Account {

    private BigDecimal creditLimit;

    public BigDecimal getCreditLimit() {
        return creditLimit;
    }

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

对于此模式，Hibernate 生成了以下数据库表

create table Account (
    DTYPE varchar(31) not null,
    id bigint not null,
    balance decimal(19,2),
    interestRate decimal(19,2),
    owner varchar(255),
    overdraftFee decimal(19,2),
    creditLimit decimal(19,2),
    primary key (id)
)

因此，当插入两个子类实体时

DebitAccount debitAccount = new DebitAccount();
debitAccount.setId( 1L );
debitAccount.setOwner( "John Doe" );
debitAccount.setBalance( BigDecimal.valueOf( 100 ) );
debitAccount.setInterestRate( BigDecimal.valueOf( 1.5d ) );
debitAccount.setOverdraftFee( BigDecimal.valueOf( 25 ) );

CreditAccount creditAccount = new CreditAccount();
creditAccount.setId( 2L );
creditAccount.setOwner( "John Doe" );
creditAccount.setBalance( BigDecimal.valueOf( 1000 ) );
creditAccount.setInterestRate( BigDecimal.valueOf( 1.9d ) );
creditAccount.setCreditLimit( BigDecimal.valueOf( 5000 ) );

Hibernate 将在 DTYPE 列中填充子类类名

INSERT INTO Account (balance, interestRate, owner, overdraftFee, DTYPE, id)
VALUES (100, 1.5, 'John Doe', 25, 'DebitAccount', 1)

INSERT INTO Account (balance, interestRate, owner, creditLimit, DTYPE, id)
VALUES (1000, 1.9, 'John Doe', 5000, 'CreditAccount', 2)

虽然这对于大多数用例来说很简单，但在集成遗留数据库架构时，辨别器列可能包含 NULL 或与任何实体子类不相关的值。

假设我们的数据库包含如下记录

INSERT INTO Account (DTYPE, balance, interestRate, owner, id)
VALUES (NULL, 300, 0.9, 'John Doe', 3)

INSERT INTO Account (DTYPE, active, balance, interestRate, owner, id)
VALUES ('Other', true, 25, 0.5, 'Johnny Doe', 4)

INSERT INTO Account (DTYPE, active, balance, interestRate, owner, id)
VALUES ('Unsupported', false, 35, 0.6, 'John Doe Jr.', 5)

使用之前的映射，当 尝试获取所有 Account(s)

Map<Long, Account> accounts = entityManager.createQuery(
        "select a from Account a", Account.class )
.getResultList()
.stream()
.collect( Collectors.toMap( Account::getId, Function.identity()));

我们会遇到以下问题

org.hibernate.WrongClassException: Object [id=3] was not of the specified subclass
[org.hibernate.userguide.inheritance.Account] : Discriminator: null

org.hibernate.WrongClassException: Object [id=4] was not of the specified subclass
[org.hibernate.userguide.inheritance.Account] : Discriminator: Other

org.hibernate.WrongClassException: Object [id=5] was not of the specified subclass
[org.hibernate.userguide.inheritance.Account] : Discriminator: Unsupported

幸运的是，Hibernate 允许我们通过使用 NULL 和 NOT NULL 辨别器值映射来处理这些映射。

对于 NULL 值，我们可以如下注解基类 Account 实体

@Entity(name = "Account")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorValue( "null" )
public static class Account {

    @Id
    private Long id;

    private String owner;

    private BigDecimal balance;

    private BigDecimal interestRate;

    // Getter and setter omitted for brevity
}

对于 Other 和 Unsupported 辨别器值，我们可以有一个处理所有未显式映射值的杂项实体

@Entity(name = "MiscAccount")
@DiscriminatorValue( "not null" )
public static class MiscAccount extends Account {

    private boolean active;

    public boolean isActive() {
        return active;
    }

    public void setActive(boolean active) {
        this.active = active;
    }
}

这样，上述多态查询就可以正常工作，我们甚至可以验证结果

assertEquals(5, accounts.size());
assertEquals( DebitAccount.class, accounts.get( 1L ).getClass() );
assertEquals( CreditAccount.class, accounts.get( 2L ).getClass() );
assertEquals( Account.class, accounts.get( 3L ).getClass() );
assertEquals( MiscAccount.class, accounts.get( 4L ).getClass() );
assertEquals( MiscAccount.class, accounts.get( 5L ).getClass() );

我还更新了 Hibernate 5.0、5.1 和 5.2 文档中的这两个非常有用的映射选项。

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