从数据库向导 jpa 2.1 @ManyToMany 创建实体，不需要修复列表

Question

我有一些像 multimedia_feature 这样的连接表。

向导将在 Multimedia 类中创建一个 List 属性：

...
@JoinTable(name = "multimedia_feature", joinColumns = {
   @JoinColumn(name = "feature_oid", referencedColumnName = "oid")}, inverseJoinColumns = {
   @JoinColumn(name = "multimedia_oid", referencedColumnName = "oid")})
@ManyToMany
private List<Feature> featureList;
...

因为我只需要一个多对一关系（一个功能可以有多个多媒体文件），我将 media_oid 标记为唯一。 在此之后，向导创建其他 2 个表（我认为是多余的）

@Entity
@Table(name = "multimedia_feature")
@XmlRootElement
@NamedQueries({
    @NamedQuery(name = "MultimediaFeature.findAll", query = "SELECT m FROM MultimediaFeature m"),
    @NamedQuery(name = "MultimediaFeature.findByMultimediaOid", query = "SELECT m FROM MultimediaFeature m WHERE m.multimediaFeaturePK.multimediaOid = :multimediaOid"),
    @NamedQuery(name = "MultimediaFeature.findByFeatureOid", query = "SELECT m FROM MultimediaFeature m WHERE m.multimediaFeaturePK.featureOid = :featureOid")})
public class MultimediaFeature implements Serializable {

    private static final long serialVersionUID = 1L;
    @EmbeddedId
    protected MultimediaFeaturePK multimediaFeaturePK;
    @JoinColumn(name = "multimedia_oid", referencedColumnName = "oid", insertable = false, updatable = false)
    @OneToOne(optional = false)
    private Multimedia multimedia;
    @JoinColumn(name = "feature_oid", referencedColumnName = "oid", insertable = false, updatable = false)
    @ManyToOne(optional = false)
    private Feature feature;
...
...

和

@Embeddable
public class MultimediaFeaturePK implements Serializable {

    @Basic(optional = false)
    @NotNull
    @Column(name = "multimedia_oid")
    private int multimediaOid;
    @Basic(optional = false)
    @NotNull
    @Column(name = "feature_oid")
    private int featureOid;
    ...
    ...

最后它在多媒体类中添加了一个属性：

....
@OneToOne(cascade = CascadeType.ALL, mappedBy = "multimedia")
    private MultimediaFeature multimediaFeature;
....

因为我有很多加入类，我会避免创建所有这些类。 我可以手动创建属性吗，例如：

@JoinTable(name = "multimedia_feature",
           @JoinColumn(name"feature_oid", referencedColumnName = "oid")
)
    @OneToOne(optional = false)
    private Feature feature;

或者这会妨碍正确的持久性？

Answer 1

看起来Multimedia类中的feature属性应该是@ManyToOne关系。 默认情况下，为多对多关系和单向一对多关系的映射创建连接表。 如果你想避免加入类，我认为你可以像这样使用@JoinTable 来映射要素类中的多媒体属性：

@OneToMany
@JoinTable(name = "multimedia_feature",
    joinColumns = @JoinColumn(name = "feature_oid"),
    inverseJoinColumns = @JoinColumn(name = "multimedia_oid") )
private List<Multimedia> multimediaList;

如果您确实需要与连接表的双向关系，映射将是这样的：

public class Feature implements Serializable {
    @OneToMany(mappedBy="feature")
    private List<Multimedia> multimediaList;
    ...
}

public class Multimedia implements Serializable {
    @ManyToOne
    @JoinTable(name = "multimedia_feature",
            joinColumns = @JoinColumn(name = "multimedia_oid") ,
            inverseJoinColumns = @JoinColumn(name = "feature_oid"))
    private Feature feature;
    ...
}

或者您可以通过在多媒体表中引入一个连接列（如 feture_oid）来完全删除具有双向关联的连接表。以便您可以轻松映射多媒体类中的特征属性：

@ManyToOne
@JoinColumn(name = "feature_oid")
private Feature feature;

在 Feature 类中会是这样的：

@OneToMany(mappedBy="feature")
private List<Multimedia> multimediaList;