Eclipselink 2.4 + JPA + ManyToMany带有附加列 [英] Eclipselink 2.4 + JPA + ManyToMany with additional column

问题描述

我正在使用Eclipselink提供的JPA实体以Java开发学生管理器应用程序。我试图在两个实体之间实现多对多关系一段时间，但遇到了一些奇怪的例外。

I'm developing a student manager application in Java with JPA entities provided by Eclipselink. I'm trying to implement a many-to-many relationship between two entities for a while, but I ran into some weird exceptions.

我的实现基于： b $ b ManyToMany关系的Java持久性Wiki页面和此StackOverflow帖子。

My implementations are based on: Java persistence Wiki page of ManyToMany relationship and this StackOverflow post.

这是我的 Homework.java 文件的相关部分：

Here is my Homework.java file's relevant parts:

@Entity
@Table(name = "db_homework")
public class Homework {
    @Id
    @GeneratedValue(strategy = IDENTITY)    
    private long id;

    ... other fields and connections ...

    @OneToMany(mappedBy = "semester")
    private Collection<SemesterHomework> semesters;

    ... getters/setters ...
}

Semester.java：

@Entity
@Table(name = "db_semester")
public class Semester {
    @Id
    @GeneratedValue(strategy = IDENTITY)    
    private long id;

    ... other fields and connections ...

    @OneToMany(mappedBy = "homework")
    private Collection<SemesterHomework> homeworks;

    ... getters/setters ...
}

SemesterHomework.java：

@Entity
@Table(name="db_semesterhomework")
@IdClass(SemesterHomeworkPK.class)
public class SemesterHomework {
    @Id
    @ManyToOne(optional=false)  
    @PrimaryKeyJoinColumn(name="HOMEWORK_ID", referencedColumnName="ID")
    private Homework homework;

    @Id
    @ManyToOne(optional=false)  
    @PrimaryKeyJoinColumn(name="SEMESTER_ID", referencedColumnName="ID")
    private Semester semester;

    @Column
    @Temporal(TemporalType.TIMESTAMP)
    private Date finishTime;
    ... getters/setters ...
}

SemesterHomeworkPK.java：

public class SemesterHomeworkPK {
    @Id
    @Column(name="SEMESTER_ID")
    private Long semester;
    @Id
    @Column(name="HOMEWORK_ID")
    private Long homework;

    @Override
    public boolean equals(Object arg0) {
        ...
    }

    @Override
    public int hashCode() {
        ...
    }

    ... getters/setters ...
}

我得到的例外：

Exception [EclipseLink-41] (Eclipse Persistence Services - 2.4.0.v20120608-r11652): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: A non-read-only mapping must be defined for the sequence number field.
Descriptor: RelationalDescriptor(org.bme.amorg.droidhfsite.server.db.Homework --> [DatabaseTable(db_homework)])

Exception [EclipseLink-41] (Eclipse Persistence Services - 2.4.0.v20120608-r11652): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: A non-read-only mapping must be defined for the sequence number field.
Descriptor: RelationalDescriptor(org.bme.amorg.droidhfsite.server.db.Semester --> [DatabaseTable(db_semester)])

Exception [EclipseLink-93] (Eclipse Persistence Services - 2.4.0.v20120608-r11652): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: The table [db_homework] is not present in this descriptor.
Descriptor: RelationalDescriptor(org.bme.amorg.droidhfsite.server.db.Semester --> [DatabaseTable(db_semester)])

Exception [EclipseLink-93] (Eclipse Persistence Services - 2.4.0.v20120608-r11652): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: The table [db_semester] is not present in this descriptor.
Descriptor: RelationalDescriptor(org.bme.amorg.droidhfsite.server.db.Homework --> [DatabaseTable(db_homework)])

当我尝试使用Eclipse JPA工具生成数据库表时，会发生异常。

The exceptions occur when I try to generate the database tables with Eclipse JPA tools.

感谢您的帮助，彼得
。

Thanks for the help in advance, Peter.

推荐答案

不确定这是唯一的问题，但您的 mappedBy 属性有误。

Not sure this is the only problem, but you got your mappedBy attributes wrong.

家庭作业中的学期列表由家庭作业 SemesterHomework 的c>字段。 学期字段除外。

The list of semesters in Homework is mapped by the homework field of SemesterHomework. Not by the semester field.

学期中的家庭作业列表由学期 SemesterHomework 的$ c>字段。 家庭作业字段除外。

And the list of homeworks in Semester is mapped by the semester field of SemesterHomework. Not by the homework field.

如果您在 SemesterHomework 中拥有非合成的自动生成的ID，一切都会变得容易得多。

Everything would be much easier if you had a non-composite, autogenerated ID in SemesterHomework.

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