在哪种情况下，您使用JPA @JoinTable批注? [英] In which case do you use the JPA @JoinTable annotation?

问题描述

在哪种情况下，您使用JPA @JoinTable批注?

In which case do you use the JPA @JoinTable annotation?

推荐答案

编辑2017-04-29 :正如一些评论者所指出的，JoinTable示例不需要mappedBy注释属性.实际上，最新版本的Hibernate拒绝通过打印以下错误来启动:

EDIT 2017-04-29: As pointed to by some of the commenters, the JoinTable example does not need the mappedBy annotation attribute. In fact, recent versions of Hibernate refuse to start up by printing the following error:

org.hibernate.AnnotationException: 
   Associations marked as mappedBy must not define database mappings 
   like @JoinTable or @JoinColumn

---

假设您有一个名为Project的实体和另一个名为Task的实体，并且每个项目可以有许多任务.

---

Let's pretend that you have an entity named Project and another entity named Task and each project can have many tasks.

您可以通过两种方式为此方案设计数据库架构.

You can design the database schema for this scenario in two ways.

第一个解决方案是创建一个名为Project的表和另一个名为Task的表，并将外键列添加到名为project_id的任务表中:

The first solution is to create a table named Project and another table named Task and add a foreign key column to the task table named project_id:

Project      Task
-------      ----
id           id
name         name
             project_id

这样，就可以为任务表中的每一行确定项目.如果使用这种方法，则在您的实体类中不需要联接表:

This way, it will be possible to determine the project for each row in the task table. If you use this approach, in your entity classes you won't need a join table:

@Entity
public class Project {

   @OneToMany(mappedBy = "project")
   private Collection<Task> tasks;

}

@Entity
public class Task {

   @ManyToOne
   private Project project;

}

另一种解决方案是使用第三个表，例如Project_Tasks，并将项目和任务之间的关系存储在该表中:

The other solution is to use a third table, e.g. Project_Tasks, and store the relationship between projects and tasks in that table:

Project      Task      Project_Tasks
-------      ----      -------------
id           id        project_id
name         name      task_id

Project_Tasks表称为联接表".要在JPA中实现第二种解决方案，您需要使用@JoinTable批注.例如，为了实现单向一对多关联，我们可以这样定义实体:

The Project_Tasks table is called a "Join Table". To implement this second solution in JPA you need to use the @JoinTable annotation. For example, in order to implement a uni-directional one-to-many association, we can define our entities as such:

Project实体:

Project entity:

@Entity
public class Project {

    @Id
    @GeneratedValue
    private Long pid;

    private String name;

    @JoinTable
    @OneToMany
    private List<Task> tasks;

    public Long getPid() {
        return pid;
    }

    public void setPid(Long pid) {
        this.pid = pid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Task> getTasks() {
        return tasks;
    }

    public void setTasks(List<Task> tasks) {
        this.tasks = tasks;
    }
}

Task实体:

Task entity:

@Entity
public class Task {

    @Id
    @GeneratedValue
    private Long tid;

    private String name;

    public Long getTid() {
        return tid;
    }

    public void setTid(Long tid) {
        this.tid = tid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

这将创建以下数据库结构:

This will create the following database structure:

@JoinTable注释还允许您自定义联接表的各个方面.例如，是否我们对tasks属性进行了如下注释:

The @JoinTable annotation also lets you customize various aspects of the join table. For example, had we annotated the tasks property like this:

@JoinTable(
        name = "MY_JT",
        joinColumns = @JoinColumn(
                name = "PROJ_ID",
                referencedColumnName = "PID"
        ),
        inverseJoinColumns = @JoinColumn(
                name = "TASK_ID",
                referencedColumnName = "TID"
        )
)
@OneToMany
private List<Task> tasks;

结果数据库将变为:

最后，如果要为多对多关联创建架构，则使用联接表是唯一可用的解决方案.

Finally, if you want to create a schema for a many-to-many association, using a join table is the only available solution.

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