java 休眠一对零或一映射
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Hibernate one to zero or one mapping
提问by Boden
I'm trying to map a one to "zero or one" relationship in Hibernate. I think I may have found a way using a many-to-one.
我试图在 Hibernate 中映射一个到“零或一”的关系。我想我可能已经找到了一种使用多对一的方法。
class A {
private B b;
// ... getters and setters
}
class B {
private A a;
}
Class A's mapping specifies:
A 类的映射指定：
<many-to-one name="b" class="B"
insert="false" update="false"
column="id" unique="true"/>
and Class B's mapping specifies:
和 B 类的映射指定：
<one-to-one name="a" class="A" constrained="true"/>
What I would like is for b to be null when no matching row for B was found in the database. So I can do this (in class A):
当在数据库中找不到与 B 匹配的行时，我希望 b 为空。所以我可以这样做（在 A 类中）：
if (b == null)
However, it seems that b is never null.
但是，似乎 b 永远不会为空。
What can I do about this?
我该怎么办？
采纳答案by Boden
The answer was to add not-found="ignore" to the many-to-one statement in A:
答案是在 A 中的多对一语句中添加 not-found="ignore"：
<many-to-one name="b" class="B" not-found="ignore" insert="false" update="false" column="id" unique="true"/>
I tried simply adding lazy="false" to B as Rob H recommended, but that resulted in a HibernateObjectRetrievalFailureException everytime I loaded an A that had no B.
我尝试按照 Rob H 的建议简单地将 lazy="false" 添加到 B，但是每次我加载没有 B 的 A 时都会导致 HibernateObjectRetrievalFailureException。
See this thread for more information:
请参阅本主题以获取更多信息：
https://forum.hibernate.org/viewtopic.php?p=2269784&sid=5e1cba6e2698ba4a548288bd2fd3ca4e
https://forum.hibernate.org/viewtopic.php?p=2269784&sid=5e1cba6e2698ba4a548288bd2fd3ca4e
回答by Tony
Like Boden said, the answer is to add not-found="ignore"to the many-to-one statement in A. Doing this with annotation:
就像 Boden 说的，答案是添加not-found="ignore"到 A 中的多对一语句中。用注解来做到这一点：
In Class A:
在 A 类中：
@ManyToOne
@Cascade({ CascadeType.ALL })
@JoinColumn(name = "Id")
@NotFound(action=NotFoundAction.IGNORE)
private B b
in Class B:
B类：
@Id
@GeneratedValue(generator = "myForeignGenerator")
@org.hibernate.annotations.GenericGenerator(
name = "myForeignGenerator",
strategy = "foreign",
parameters = @Parameter(name = "property", value = "a")
)
private Long subscriberId;
@OneToOne(mappedBy="b")
@PrimaryKeyJoinColumn
@NotFound(action=NotFoundAction.IGNORE)
private A a;
回答by Rob H
Try setting lazy="false" on the many-to-one element. That should force Hibernate to try to fetch the association ("B") when the first object ("A") is loaded. The property in "A" will either be initialized with a real instance of "B" or null.
尝试在多对一元素上设置 lazy="false"。这应该会强制 Hibernate 在加载第一个对象（“A”）时尝试获取关联（“B”）。“A”中的属性将使用“B”的真实实例或 null 进行初始化。
