Hibernate - 为什么使用多对一来表示一对一？ [英] Hibernate - why use many-to-one to represent a one-to-one?

问题描述

我见过人们使用多对一的映射来表示一对一的关系。我还在Gavin King的一本书和文章中阅读了这些内容。

例如，如果客户可以只有一个送货地址，并且送货地址可以属于只给一个客户，映射如下：

 < class name =Customertable =CUSTOMERS> 
 ... 
<多对一名称=shippingAddress
 class =地址
 column =SHIPPING_ADDRESS_ID
 cascade =save-更新
 unique =true/> 
 ... 
< / class> 
  

本书原因如下（引用它）：

您不关心关联的目标端是什么，因此您可以像 to-one 关联那样对待它，而不用使用 / b>

我的问题是，为什么要使用多对一而不是一对一？什么是一对一，这对于多对一来说是不太理想的选择？

谢谢。

解决方案

数据库中的一对一关联：您可以共享主键，但也可以使用具有唯一约束的外键关系（一个表具有引用关联表的主键的外键列）。

在后面的例子中，映射它的hibernate方法是使用多对一关联它允许指定外键）。

lockquote

原因很简单：你不关心
目标端有什么的
协会，所以你可以把它看作
a to-one协会，而不需要很多
部分。所有你想要表达的是这个
实体有一个属性是
引用另一个
实体的实例，并且使用一个外键字段
表示

换句话说，使用多对一是映射一对一外键关联（实际上可能比共享主键一对一关联更频繁）的方式。

I've seen people use many-to-one mappings to represent one-to-one relationships. I've also read this in a book by Gavin King and on articles.

For example, if a customer can have exactly one shipping address, and a shipping address can belong to only one customer, the mapping is given as:

<class name="Customer" table="CUSTOMERS">
    ...
    <many-to-one name="shippingAddress"
                 class="Address"
                 column="SHIPPING_ADDRESS_ID"
                 cascade="save-update"
                 unique="true"/>
    ...
</class>

The book reasons as (quoting it):

"You don't care what's on the target side of the association, so you can treat it like a to-one association without the many part."

My question is, why use many-to-one and not one-to-one? What is it about a one-to-one that makes it a less desirable option to many-to-one?

Thanks.

解决方案

There are several ways to implement a one-to-one association in a database: you can share a primary key but you can also use a foreign key relationship with a unique constraint (one table has a foreign key column that references the primary key of the associated table).

In the later case, the hibernate way to map this is to use a many-to-one association (that allows to specify the foreign key).

The reason is simple: You don’t care what’s on the target side of the association, so you can treat it like a to-one association without the many part. All you want is to express "This entity has a property that is a reference to an instance of another entity" and use a foreign key field to represent that relationship.

In other words, using a many-to-one is the way to map one-to-one foreign key associations (which are actually maybe more frequent than shared primary key one-to-one associations).

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