java @MappedSuperclass 不是 JPA 中的 @Entity 吗？
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@MappedSuperclass is not an @Entity in JPA?
提问by Rakim
So I am using DerbyDB and I am setting up some entities. I have a @MappedSuperclasswhich is used as a superclass for some entities (@Entity). More specifically, I have a superclass Userand 3 subclasses namely admin, regularand guest. Now I have a different entity, let's say filethat should reference (as one of its fields) its owner. So I created a field called User owner.
The error I get is:
所以我正在使用 DerbyDB 并且我正在设置一些实体。我有一个@MappedSuperclass用作某些实体 ( @Entity)的超类。更具体地说，我有一个超类User和 3 个子类，即admin,regular和guest。现在我有一个不同的实体，假设file它应该引用（作为其字段之一）其所有者。所以我创建了一个名为User owner. 我得到的错误是：
Exception Description: [File] uses a non-entity [User] as target entity in the relationship attribute [field owner].
Is there a workaround?
有解决方法吗？
回答by XiCoN JFS
I can suggest two solutions:
我可以建议两种解决方案：
Change Inheritance
改变继承
The exception you get clearly describes your problem: Useris not an entity. Any class declared as superclass with the interface @MappedSuperclasscannot be an entity (in standard JPA - depends on your JPA-provider)... let me point you to an answer I just gave to quite a similar problem
您得到的异常清楚地描述了您的问题：User不是实体。任何声明为具有接口的超类的类@MappedSuperclass都不能是实体（在标准 JPA 中 - 取决于您的 JPA 提供者）……让我指出我刚刚对一个非常相似的问题给出的答案
--> Superclass-Types
-->超类类型
So defining your superclass as an abstract entity will give you the desired behaviour, you described.
因此，您描述的将超类定义为抽象实体将为您提供所需的行为。
Extra:
额外的：
If you choose your inheritance mapping strategy as @Inheritance(strategy = InheritanceType.SINGLE_TABLE)you don't even need multiple database-tables. Here is a good example: JPA Single-Table Inheritance
如果您选择继承映射策略，因为@Inheritance(strategy = InheritanceType.SINGLE_TABLE)您甚至不需要多个数据库表。这是一个很好的例子：JPA 单表继承
Change Model (suggested)
更改模型（建议）
Don't split your user entity in several entities just by their roles. Make an Enumwith all your desired roles and add it as a field to your User-entity. This is widely more common, unless u need your admin, guests etc to be an own object...
不要仅根据角色将您的用户实体拆分为多个实体。做一个Enum与所有您想要的角色，并将其添加为一个字段，你User-entity。这更常见，除非你需要你的管理员、客人等成为自己的对象......
回答by Vlad Mihalcea
As I explained in this article, @MappedSupperclassis different than the @Inheritanceannotation.
正如我在解释这个文章，@MappedSupperclass是不是不同的@Inheritance注解。
@MappedSuperclasstells the JPA provider to include the base class persistent properties as if they were declared by the child class extending the superclass annotated with @MappedSuperclass.
@MappedSuperclass告诉 JPA 提供者包含基类持久属性，就好像它们是由扩展用 注释的超类的子类声明的@MappedSuperclass。
However, the inheritance is only visible in the OOP world, since, from a database perspective, there's no indication of the base class. Only the child class entity will have an associated mapped table.
然而，继承只在 OOP 世界中可见，因为从数据库的角度来看，没有基类的指示。只有子类实体才会有关联的映射表。
The @Inheritanceannotation is meant to materialize the OOP inheritance model in the database table structure. More, you can query a base class annotated with @Inheritancebut you can't do that for a base class annotated with @MappedSuperclass.
该@Inheritance注释是为了兑现在数据库表结构的OOP继承模型。此外，您可以查询带有注释的基类，@Inheritance但不能查询带有注释的基类@MappedSuperclass。
