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 table（剧集 table 中的 programmeId 可能因为连接 table 而多余？）。我得出了以下关系，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，是否正确？

@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 table 是固定的，你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，剧集有一个 ManyToOne 回到程序使用 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 关系控制着数据库中的外键，是否要实际添加Programme.episodes 集合由您决定。如果这样做，您必须自己维护它，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节目集时将其添加到节目集合中。

至于 prog_ep_perf table，这是一个可能难以处理的 3 向映射。如果要为每一行填充所有 3 个外键，那么它不能按照您的布局方式进行映射，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 table 映射为一个实体，这样您就可以在应用程序中完全按照您的意愿或需要来处理它。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 for additional information (JPA 1.0) and for JPA 2.0 see https://wiki.eclipse.org/EclipseLink/Examples/JPA/2.0/MapKeyColumns and 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;
  ..
}

映射 JPA 实体

Mapping JPA Entities

java

hibernate

jpa