inheritance mapping

最新推荐文章于 2016-11-30 17:03:39 发布

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最新推荐文章于 2016-11-30 17:03:39 发布

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本文介绍了三种常见的对象关系映射(ORM)策略：按类层次结构表(Table per class hierarchy)、按子类表(Table per subclass)及按具体类表(Table per concrete class)。以支付接口为例，展示了按类层次结构表的具体实现方式及其限制。

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the three strategies:
[list]
[*]table per class hierarchy
[*]table per subclass
[*]table per concrete class
[/list]

[b]1.Table per class hierarchy[/b]
Suppose we have an interface Payment with the implementors CreditCardPayment and CashPayment. The table per class hierarchy mapping would display in the following way:


<class name="Payment" table="PAYMENT">
<id name="id" type="long" column="PAYMENT_ID">
<generator class="native"/>
</id>
<discriminator column="PAYMENT_TYPE" type="string"/>
<property name="amount" column="AMOUNT"/>
...
<subclass name="CreditCardPayment" discriminator-value="CREDIT">
<property name="creditCardType" column="CCTYPE"/>
...
</subclass>
<subclass name="CashPayment" discriminator-value="CASH">
...
</subclass>
</class>

Exactly one table is required. There is one limitation of this mapping:columns declared by the subclass cannot have [i]not null [/i]constraints.