Eclipselink 2.4 + JPA + ManyToMany附加列

我正在用Java开发一个学生管理器应用程序，使用Eclipselink提供的JPA实体。我试图在一段时间内实现两个实体之间的多对多关系，但是我遇到了一些奇怪的例外情况。Eclipselink 2.4 + JPA + ManyToMany附加列

我的实现基于： Java persistence Wiki page of ManyToMany relationship和this StackOverflow post。

这里是我的Homework.java文件的相关部分：

@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 ... 
}

的例外我所得到的：当我尝试生成与Eclipse JPA工具数据库表

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)])

例外情况发生。

感谢您的帮助，彼得。

来源

2012-10-28 nothingam

不确定这是唯一的问题，但你的mappedBy属性错误。

Homework的学期清单由homework字段SemesterHomework映射。不由semester字段。

并且Semester中的作业列表由semester字段SemesterHomework映射。不是由homework字段。

如果您在SemesterHomework中有非复合自动生成的ID，那么一切都会更容易。

来源

2012-10-28 14:34:43

Eclipselink 2.4 + JPA + ManyToMany附加列

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