Hibernate——为什么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映射来表示一对一关系.我还在 Gavin King 的一本书和文章中读到了这一点.

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.

例如，如果一个客户只能有一个送货地址，而一个送货地址只能属于一个客户，则映射如下:

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>

这本书的理由是(引用它):

The book reasons as (quoting it):

您不关心关联的目标端是什么，因此您可以将其视为 对一 关联而无需 many 部分."

"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."

我的问题是，为什么使用 many-to-one 而不是 one-to-one?一对一是什么使它成为多对一的不太理想的选择?

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?

谢谢.

推荐答案

在数据库中实现一对一关联的方法有多种:可以共享主键，也可以使用外键关系唯一约束(一个表具有引用关联表主键的外键列).

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).

在后一种情况下，hibernate 映射它的方法是使用 多对一 关联(允许指定外键).

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).

原因很简单:你不在乎目标端是什么关联，所以你可以把它当作没有很多的一对一关联部分.你只想表达这实体有一个属性是引用另一个实例实体"并使用外键字段代表这种关系.

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.

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<强>共享主键一对一关联).

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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