JPA多态的oneToMany [英] JPA polymorphic oneToMany

问题描述

我不知道如何使用JPA干净地制作标签云，其中每个数据库实体可以具有许多标签.

I couldn't figure out how to cleanly do a tag cloud with JPA where each db entity can have many tags.

例如

帖子可以有0个或更多标签 用户可以有0个或更多标签

Post can have 0 or more Tags User can have 0 or more Tags

在JPA中，是否有比必须使所有实体子类(例如Taggable抽象类)更好的方法?标签实体引用多个标签的地方.

Is there a better way in JPA than having to make all the entities subclass something like Taggable abstract class? Where a a Tag entity would reference many Taggables.

标签云只是一个示例，用于简化我遇到的问题.在我的场景中，该关系应为OneToMany，其中标签无法重用.

the tag cloud is just a sample to simplify the problem I am having. In my scenario, the relation should be OneToMany where a Tag cannot be reused.

谢谢

推荐答案

在JPA中，有没有比使所有实体成为子类(如Taggable抽象类)更好的方法?

Is there a better way in JPA than having to make all the entities subclass something like Taggable abstract class?

让我们忘记这个示例:) JPA确实支持多态关联，但是目标类必须是继承层次结构的一部分.以下是有关继承策略的更多经验法则:

Let's forget the example :) JPA does support polymorphic associations but the target classes have to be part of an inheritance hierarchy. And here are some more rules of thumb about inheritance strategies:

• SINGLE_TABLE:
  • 层次结构中的所有类都映射到单个表
  • 此策略为之间的多态关系提供了良好的支持 涵盖以下内容的实体和查询 整个类的层次结构.
  • 可能包含某些子类数据的空字段

• SINGLE_TABLE:
  • All the classes in a hierarchy are mapped to a single table
  • This strategy provides good support polymorphic relationships between entities and queries that cover the entire class hierarchy.
  • May contain null fields for some subclass data

• 层次结构中的每个类都映射到单独的表，因此提供了 对多态性的支持不佳 关系
• 为每个子类需要SQL联合或单独的SQL查询

• Each class in a hierarchy mapped to a separate table and hence, provides poor support for polymorphic relationships
• requires SQL union or separate SQL queries for each subclass

• 没有空字段=>紧凑数据
• 这为多态关系提供了良好的支持，但是 需要一个或多个联接操作– 可能会导致效果不佳

• no null fields => compact data
• This provides good support for polymorphic relationships, but requires one or more join operations – may result in poor performance

简而言之，如果子类声明的属性相对较少，则首选SINGLE_TABLE策略.如果不是这样，请使用JOINED策略，除非您具有较深的层次结构(在这种情况下，连接的成本可能比联合的成本更高，然后TABLE_PER_CLASS的情况会更糟").

In short, if your subclasses declare relatively few properties, prefer the SINGLE_TABLE strategy. If not, use a JOINED strategy unless you have a deep hierarchy (in which case the cost of joins may become more expensive than unions and then TABLE_PER_CLASS would be "less worse").

• JPA 1.0规范
  • 第2.1.9节继承"
  • 第2.1.10节"2.1.10继承映射策略"
  • JPA 1.0 Specification
    • Section 2.1.9 "Inheritance"
    • Section 2.1.10 "2.1.10 Inheritance Mapping Strategies"

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