Hibernate映射:一列到多个表

Question

我有一个场景的"最佳实践"问题.

场景:数据库中的多个实体,例如Document,BlogPost,Wiki可以由个人共享.不是为每个实体创建共享表,而是创建单个共享表.问题是,如何使用不同的实体映射共享表？

我有三个选项,请告知哪个选项最好,以及是否有更好的选择.

选项1: 创建表共享为:

SHARES  
id (unique)
entityId (non DB enforced FK to DOCUMENTS, WIKIS, POSTS etc.)
entityType
sharedBy
sharedWith
sharedDate

这里,entityId将是documentId,wikiId,postId等的FK,entityType将识别entityId的类型.

在创建共享到实体映射时,这在Hibernate建模中存在问题,例如share.getDocument()或share.getWiki()等.

选项2: 创建仅包含共享信息的表共享,然后创建将共享绑定到实体的解析表.

SHARES
id(PK)
sharedBy
sharedWith
sharedDate
shareType (helper field for searches)

SHARES_DOCUMENTS
share_id (unique ID and FK, one to one with SHARES)
document_id (FK to DOCUMENTS)

SHARES_POST
share_id (unique ID and FK, one to one with SHARES)
post_id (FK to POSTS)

more share tables here.

因此,hibernate明智地,Share可以为每个共享类型一对一(例如share.getDocument(),share.getPost(),并且shareType将标识哪个关系是'活动')

选项3 与选项1类似,但创建单个列而不是实体ID

SHARES
id (unique ID)
documentId (FK to DOCUMENTS, nullable)
postId (FK to POSTS, nullable)
wikiId (FK to WIKIS, nullable)
sharedBy
sharedWith
sharedDate
sharedType

这里,每列可以映射到相应的实体,但它们可以为空.sharedType可以识别哪个关系是"活动的".

所以,问题是,哪种做法最好,既有数据库方面也有hibernate映射(最终查询,性能方面).

谢谢M.相反

Answer 1

正如 TheStijn 所建议的那样，在研究了设置继承关系的不同方法后，我采用了“每个类层次结构单个表”的方法，最终得到的表如下：

SHARES
---------
id PK
shared_by FK to User
shared_with FK to User
shared_Date
document_id nullable FK to Document
post_id nullable FK to Posts
... more ids here to link to more entities
type_discriminator (values, DOCUMENT, POST ... )

在 Hibernate/Java 方面，One Share 抽象类作为...

@Entity
@Table(name="SHARES")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="TYPE_DISCRIMINATOR", discriminatorType=DiscriminatorType.STRING)
public abstract class Share {
    @Id
    @Column( name="ID", nullable=false )
    @GeneratedValue(generator="system-uuid")
    @GenericGenerator(name="system-uuid", strategy = "uuid")
    private String id;

    @ManyToOne
    @JoinColumn( name="SHARED_BY", nullable=false )
    private User sharedBy;

    @ManyToOne
    @JoinColumn( name="SHARED_WITH", nullable=false )
    private User sharedWith;

    @Column(name="SHARED_DATE", columnDefinition="TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP", nullable=false)
    @Temporal(TemporalType.TIMESTAMP)
    private Date sharedDate;        
    ...

}

还有两个普通班..

@Entity
@DiscriminatorValue("DOCUMENT")
public class SharedDocument extends Share { 
    @ManyToOne
    @JoinColumn( name="DOCUMENT_ID", nullable=true )
    private Document document;
    ....

}

@Entity
@DiscriminatorValue("POST")
public class SharedPost extends Share {
    @ManyToOne
    @JoinColumn( name="POST_ID", nullable=true )
    private Post post;
    ....

}

至于用法，仅将具体类用作：

@Test
public void saveNewDocumentShare(){
    SharedDocument sharedDocument = new SharedDocument();
    sharedDocument.setDocument(document1);
    sharedDocument.setSharedBy(teacher1);
    sharedDocument.setSharedWith(teacher2);
    sharedDocument.setSharedDate(new Date());

    sharedDocument.setCreatedBy("1");
    sharedDocument.setCreatedDate(new Date());
    sharedDocument.setModifiedBy("1");
    sharedDocument.setModifiedDate(new Date());


    SharedDocument savedSharedDocument = dao.saveSharedDocument(sharedDocument);

    assertNotNull(savedSharedDocument);
    assertThat(savedSharedDocument.getId(),notNullValue());
}

@Test
public void saveNewPostShare(){
    SharedPost sharedWikiPage = new SharedWikiPage();
    sharedPost.setPost(post1);
    sharedPost.setSharedBy(teacher1);
    sharedPost.setSharedWith(teacher2);
    sharedPost.setSharedDate(new Date());

    sharedPost.setCreatedBy("1");
    sharedPost.setCreatedDate(new Date());
    sharedPost.setModifiedBy("1");
    sharedPost.setModifiedDate(new Date());


    SharedPost savedSharedPost = dao.saveSharedPost(sharedPost);

    assertNotNull(savedSharedPost);
    assertThat(savedSharedPost.getId(),notNullValue());

}