Java @ManyToOne JPA 关系可以为空吗？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/25718229/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Can a @ManyToOne JPA relation be null?
提问by Narges
I have a table that has foreign key of another table (many to one relationship) but i want it to be nullable.
我有一个表，它有另一个表的外键（多对一关系），但我希望它可以为空。
Something like this:
像这样的东西：
public class SubType() {
@Id
@GeneratedValue(generator = "system-uuid")
@GenericGenerator(name = "system-uuid", strategy = "uuid")
private String id;
}
public class TopUp {
@Column(nullable = true)
@ManyToOne(optional = false, fetch = FetchType.LAZY)
private SubType subType;
}
But @Column(nullable = true)throws the NullPointerExceptionand says subtype can not be null.
Is there any way to get the ManyToOne accept null?
但是@Column(nullable = true)抛出NullPointerException并说子类型不能为空。有没有办法让 ManyToOne 接受 null？
采纳答案by Vlad Mihalcea
You need to set:
您需要设置：
@ManyToOne(optional = true, fetch = FetchType.LAZY)
not optional=false.
不是optional=false。
The @Column(nullable=true)is to instruct the DDL generation tool to include a NULLSQL column type constraint.
该@Column(nullable=true)是指示DDL生成工具，包括NULLSQL列类型约束。
For more on optionalvs nullable, check out this StackOverflow answer.
有关optionalvs 的更多信息nullable，请查看此 StackOverflow 答案。
回答by Sarwar kamal
try this:
尝试这个：
@JoinColumn(name = "subType_id", nullable = true)
回答by Oleksandr_DJ
Only this helped me:
只有这对我有帮助：
public class TopUp {
@ManyToOne
@JoinColumn(columnDefinition="integer", name="subtype_id")
private SubType subType;
}
columnDefinition is a solution!
columnDefinition 是一个解决方案！
