JPA relations

傷城~ 2022-06-16 12:54 99阅读 0赞

# JPA 中实体之间的关系 #

一共有如下四种关系  
\- @ManyToOne  
\- @OneToMany  
\- @OneToOne  
\- @ManyToMany

## ManyToOne (多对一) ##

> Many-To-One relation between entities: Where one entity (column or set of columns) is/are referenced with another entity (column or set of columns) which contain unique values. In relational databases these relations are applicable by using foreign key/primary key between tables.

例：  
From Employee to Department

多个员工属于同一个部门

## OneToMany （一对多） ##

> In a one-to-many relationship between Table A and Table B, each row in Table A is linked to 0, 1 or many rows in Table B.

例：  
From Department to Employee

一个部门可以有多个员工

## OneToOne (一对一) ##

> In One-To-One relationship, one item can belong to only one other item. It means each row of one entity is referred to one and only one row of another entity.

例：  
Department : Employee

一个部门对应一个员工

## ManyTOMany (多对多) ##

> Many-To-Many relationship is where one or more rows from one entity are associated with more than one row in other entity.

例：  
Class : Teacher

在双向关系中，多个班级对应多个老师，多个老师对应多个班级。

此时会生成一张中间表 Teacher\_Class

## @JoinColumn ##

> A Join Column in JPA is a column in the owner entity that refers to a key (usually a primary key) in the non-owner or inverse entity. The first thing that comes to mind after reading the above line is that JoinColumns are nothing but a Foreign Key Columns. Another thing to notice above is that we have used the terms owner and non-owner entities. For easy rememberability, the entity that has a join column is always the owning entity.

*  ManyToOne mappings are always defined on the owning entity.

参考  
[1][]  
[2][]

[1]: https://www.tutorialspoint.com/jpa/jpa_entity_relationships.htm
[2]: http://www.kumaranuj.com/2013/05/jpa-2-understanding-relationships.html