JPA @ElementCollection不会创建列

Question

在我的应用程序中，我有@Entity 嵌入一个Request类，其中包含Issue属性和Attachment个列表。
Attachment也是Issue Issue Code属性为number和year，因此我有：

@Embeddable
public class Code{
     private Long number;
     private Integer year;
}

@Embeddable
public class Issue{
    private Code code;
}

@Embeddable
public class Attachment extends Issue{
     @ManyToOne
     private Category category;
}

@Embeddable
public class Request{
    @AttributeOverrides(value={
        @AttributeOverride(name="code.year", column=@Column(name="issue_year")),
        @AttributeOverride(name="code.number", column=@Column(name="issue_number")),
    })
    private Issue issue;

    @AttributeOverrides(value={
        @AttributeOverride(name="code.year", column=@Column(name="attachment_year")),
        @AttributeOverride(name="code.number", column=@Column(name="attachment_number")),
    })
    @ElementCollection
    @CollectionTable(name="attachments")
    private List<Attachment> attachments;
}

现在的问题是，在生成的表中，我有attachments表但没有attachment_year和attachment_number列！

我做错了什么？