一对多、多对一和多对多之间的区别? [英] Difference Between One-to-Many, Many-to-One and Many-to-Many?

问题描述

好的，所以这可能是一个微不足道的问题，但我无法想象和理解差异以及何时使用每个差异.我也有点不清楚像单向和双向映射这样的概念如何影响一对多/多对多关系.我现在正在使用 Hibernate，因此任何与 ORM 相关的解释都会有所帮助.

Ok so this is probably a trivial question but I'm having trouble visualizing and understanding the differences and when to use each. I'm also a little unclear as to how concepts like uni-directional and bi-directional mappings affect the one-to-many/many-to-many relationships. I'm using Hibernate right now so any explanation that's ORM related will be helpful.

举个例子，假设我有以下设置:

As an example let's say I have the following set-up:

public class Person{
    private Long personId;
    private Set<Skill> skills;
    //Getters and setters
}

public class Skill{
    private Long skillId;
    private String skillName;
    //Getters and setters
}

那么在这种情况下，我会有什么样的映射?绝对感谢对这个特定示例的回答，但我也非常想了解何时使用一对多和多对多，以及何时使用连接表与连接列以及单向与双向.

So in this case what kind of mapping would I have? Answers to this specific example are definitely appreciated but I would also really like an overview of when to use either one-to-many and many-to-many and when to use a join table versus a join column and unidirectional versus bidirectional.

推荐答案

一对多:一个人有很多技能，一个技能不会在人之间重复使用

One-to-Many: One Person Has Many Skills, a Skill is not reused between Person(s)

• 单向:一个人可以通过它的集合直接引用技能
• 双向:每个子"技能都有一个指向人(未显示在您的代码中)

• Unidirectional: A Person can directly reference Skills via its Set
• Bidirectional: Each "child" Skill has a single pointer back up to the Person (which is not shown in your code)

Many-to-Many:一个人有很多技能，一个技能在人之间重复使用

Many-to-Many: One Person Has Many Skills, a Skill is reused between Person(s)

• 单向:一个人可以通过它的集合直接引用技能
• 双向:一项技能有一组与之相关的人员.

• Unidirectional: A Person can directly reference Skills via its Set
• Bidirectional: A Skill has a Set of Person(s) which relate to it.

在一对多关系中，一个对象是父"，一个是子".父母控制孩子的存在.在多对多中，任何一种类型的存在都依赖于它们之外的某些东西(在更大的应用程序上下文中).

In a One-To-Many relationship, one object is the "parent" and one is the "child". The parent controls the existence of the child. In a Many-To-Many, the existence of either type is dependent on something outside the both of them (in the larger application context).

您的主题(领域)应该决定关系是一对多还是多对多——但是，我发现使关系单向或双向是一个需要牺牲记忆的工程决策，处理、性能等

Your subject matter (domain) should dictate whether or not the relationship is One-To-Many or Many-To-Many -- however, I find that making the relationship unidirectional or bidirectional is an engineering decision that trades off memory, processing, performance, etc.

令人困惑的是，多对多双向关系不需要对称！也就是说，一群人可以指向一项技能，但该技能不一定只与那些人相关.通常它会，但这种对称性不是必需的.以爱为例——它是双向的(我爱"、爱我")，但通常是不对称的(我爱她，但她不爱我")！

What can be confusing is that a Many-To-Many Bidirectional relationship does not need to be symmetric! That is, a bunch of People could point to a skill, but the skill need not relate back to just those people. Typically it would, but such symmetry is not a requirement. Take love, for example -- it is bi-directional ("I-Love", "Loves-Me"), but often asymmetric ("I love her, but she doesn't love me")!

所有这些都得到了 Hibernate 和 JPA 的良好支持.请记住，在管理双向多对多关系时，Hibernate 或任何其他 ORM 并没有强调保持对称性……这完全取决于应用程序.

All of these are well supported by Hibernate and JPA. Just remember that Hibernate or any other ORM doesn't give a hoot about maintaining symmetry when managing bi-directional many-to-many relationships...thats all up to the application.

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