JPA很多人都有额外的专栏

我有一个我需要解决的问题。 核心问题是我想在JPA中为JoinTable for ManyToMany关系添加额外的列。 就我而言，我有以下实体。

主题是一个简单的实体，它有许多RemoteDocument（一个RemoteDocument可能被许多主题引用，因此它应该是ManyToMany关系）。 此外，RemoteDocument实体是只读的，因为它可能只能从Oracle物化视图中读取，而且禁止更改此物化视图。 所以我想存储与某些主题相关的RemoteDocuments的顺序。 事实上，我可以使用其他实体做类似的事情：

@Entity public class Topic { @Id private Long id; @Basic private String name; @OneToMany private Set association; } @Entity public class RemoteDocument { @Id private Long id; @Basic private String description; } @Entity public class TopicToRemoteDocument { @OneToOne private Topic topic; @OneToOne private RemoteDocument remoteDocument; @Basic private Integer order; } 

在这种情况下，附加实体TopicToRemoteDocument帮助我用OneToMany替换ManyToMany关联并添加额外的字段顺序。

但我想拥有ManyToMany关系，但在连接表中配置了额外的列

Answer 1

使用list而不是set，以及@OrderColumn注释和JPA将自动处理订单：

 @MappedSuperclass public class BaseEntity{ @Id @GeneratedValue(strategy = GenerationType.AUTO) private Long id; public Long getId(){ return id; } public void setId(final Long id){ this.id = id; } } @Entity public class Topic extends BaseEntity{ @ManyToMany(mappedBy = "topics") @OrderColumn private List documents = new ArrayList(); public List getDocuments(){ return documents; } public void setDocuments(final List documents){ this.documents = documents; } } @Entity public class Document extends BaseEntity{ @ManyToMany @OrderColumn private List topics = new ArrayList(); public List getTopics(){ return topics; } public void setTopics(final List topics){ this.topics = topics; } } 

生成的DDL（使用hibernate和HSQL）：

 create table Document ( id bigint generated by default as identity (start with 1), primary key (id) ); create table Document_Topic ( documents_id bigint not null, topics_id bigint not null, topics_ORDER integer not null, documents_ORDER integer not null, primary key (documents_id, topics_ORDER) ); create table Topic ( id bigint generated by default as identity (start with 1), primary key (id) ); alter table Document_Topic add constraint FK343B5D0B481100B2 foreign key (documents_id) references Document; alter table Document_Topic add constraint FK343B5D0B558627D0 foreign key (topics_id) references Topic; 

Answer 2

我会尽量避免使用List除非你允许重复。

有一个@OrderColumn注释可以自动执行此操作。 你试过吗？

 @Entity public class Topic { @Id private Long id; @Basic private String name; @OneToMany @OrderColumn private Set association; } 

Answer 3

在创建多对多映射类实体时，一种有用的技术是将类中的id与@ManyToOne指定一起归属，这使得该类充当复合键类：

 @Entity @Table(name = "market_vendor") public class MarketVendor implements Serializable { @Id @ManyToOne @JoinColumn(name = "market_id") private Market market; @Id @ManyToOne @JoinColumn(name = "vendor_id") private Vendor vendor; @Basic @Column(name="active") private boolean active; public MarketVendor(Market market, Vendor vendor, boolean active) { this.market = market; this.vendor = vendor; this.active = active; } } 

这允许您在同一个类中定义复合主键，而无需具有单独的主键类。 您还需要使类可序列化。