java - 映射 JPA 实体

Question

我有很多 table

programme       episode         performer       prog_ep_perf
----------      ---------       ---------       ------------
programmeId     episodeId       performerId     programmeId
progTitle       episodeTitle    performerName   episodeId
                programmeId                     performerId

prog_ep_perf 是一个连接表(剧集表中的programmeId可能因为连接表而变得多余？)。我总结出了以下关系，我认为是正确的

programme
  @OneToMany
  episode     --> One programme can have many episodes
  @OneToMany
  performers  --> One programme can have many performers

episode
  @OneToOne
  programme  --> One episode links to one programme
  @OneToMany
  performers --> One episode can have many performers

performer
  @OneToMany
  programme  --> One performer can have many programmes
  @OneToMany
  episode    --> One performer can have many episodes

这是我设置实体的方式，正确吗？

@Entity
@Table(name = "PROGRAMME")
public class Programme {

  @Id
  @GeneratedValue(generator="increment")
  @GenericGenerator(name="increment", strategy="increment")
  private Long programmeId;

  private String progTitle;

  @OneToMany(      
      targetEntity=Performer.class,
      cascade={CascadeType.PERSIST, CascadeType.MERGE}
  )
  @JoinTable(
      name="PROG_EP_PERF",
      joinColumns=@JoinColumn(name="PROGRAMMEID"),
      inverseJoinColumns={@JoinColumn(name="PERFORMERID")}
  )
  private Set<Performer> performers;

  @OneToMany(      
      targetEntity=Performer.class,
      cascade={CascadeType.PERSIST, CascadeType.MERGE}
  )
  @JoinTable(
      name="PROG_EP_PERF",
      joinColumns=@JoinColumn(name="PROGRAMMEID"),
      inverseJoinColumns={@JoinColumn(name="EPISODEID")}
  )
  private Set<Episode> episodes;
}

@Entity
@Immutable
@Table(name = "EPISODE")
public class Episode {

  @Id
  @GeneratedValue(generator="increment")
  @GenericGenerator(name="increment", strategy="increment")
  private Long episodeId;

  @Type(type="com.springtests.model.Programme")
  @OneToOne( cascade = {CascadeType.PERSIST, CascadeType.MERGE} )
  @JoinColumn(name="PROGRAMMEID")
  private Programme programme;

  private String episodeTitle;

  @OneToMany(      
      targetEntity=Performer.class,
      cascade={CascadeType.PERSIST, CascadeType.MERGE}
  )
  @JoinTable(
      name="PROG_EP_PERF",
      joinColumns=@JoinColumn(name="EPISODEID"),
      inverseJoinColumns={@JoinColumn(name="PERFORMERID")}
  )
  private Set<Performer> performers;

}

@Entity
@Immutable
@Table(name = "PERFORMER")
public class Performer {

  @Id
  @GeneratedValue(generator="increment")
  @GenericGenerator(name="increment", strategy="increment")
  private Long performerId;      

  private String performerName;

  @OneToMany(      
      cascade = {CascadeType.PERSIST, CascadeType.MERGE},
      targetEntity = Programme.class
  )
  @JoinTable(
      name="PROG_EP_PERF",
      joinColumns=@JoinColumn(name="PERFORMERID"),
      inverseJoinColumns={@JoinColumn(name="PROGRAMMEID")}
  )
  public Set<Programme> programmes;

  @OneToMany(      
      cascade = {CascadeType.PERSIST, CascadeType.MERGE},
      targetEntity = Programme.class
  )
  @JoinTable(
      name="PROG_EP_PERF",
      joinColumns=@JoinColumn(name="PERFORMERID"),
      inverseJoinColumns={@JoinColumn(name="EPISODEID")}
  )
  public Set<Episode> episodes;

}

Answer 1

如果表是固定的，并且您只需要实体来表示数据库中的内容，那么看起来程序有一个 OneToMany 到剧集，并且剧集有一个 ManyToOne 返回程序，使用 Episode.programmeId 字段作为外键:

public class Programme {
  ..
  @OneToMany(mappedby="programme")
  private Set<Episode> episodes;
  ..

public class Episode {
  ..
  @ManyToOne
  @JoinColumn(name = "programmeId")
  private Programme programme;
  ..

由于 Episode.programme 关系控制数据库中的外键，因此是否要实际添加 Programme.episodes 集合取决于您。如果这样做，您必须自己维护，以便在进行更改时保持双方同步。即，如果您将剧集添加到集合中，JPA 不会自动修复节目引用，也不会在您将剧集指向该剧集时将该剧集添加到节目集合中。

对于 prog_ep_perf 表，这是一个可能难以处理的三向映射。如果要为每一行填充所有 3 个外键，则无法按照您布局的方式进行映射，因为通过将其分解为多个 1:M 映射，一个外键将始终为空。

相反，最好的解决方案是将 prog_ep_perf 表映射为实体，以便您可以在应用程序中完全按照您想要或需要的方式处理它。另一种方法是使用 Map 作为具有 ManyToMany 关系的集合类型。看 http://en.wikibooks.org/wiki/Java_Persistence/ManyToMany#Mapping_a_Join_Table_with_Additional_Columns有关更多信息 (JPA 1.0) 和 JPA 2.0，请参阅 https://wiki.eclipse.org/EclipseLink/Examples/JPA/2.0/MapKeyColumns和 http://docs.oracle.com/javaee/6/api/javax/persistence/MapKeyJoinColumn.html ，但它可能看起来像:

public class Programme {
  ..
  @ManyToMany
  @JoinTable(name="prog_ep_perf",
                   joinColumns=@JoinColumn(name="episodeId"),
                   inverseJoinColumns=@JoinColumn(name="programmeId"))
  @MapKeyJoinColumn(name="performerId")
  Map<Performer, Episode> prog_ep_perf;
  ..
}