如何自動生成連接表的GUID？

Question

我正在使用Hibernate 4.1.0.Final，JPA 2.0和MySQL 5.5。我有以下實體：

@GenericGenerator(name = "uuid-strategy", strategy = "uuid.hex") 
@Entity 
@Table(name = "cb_organization", uniqueConstraints = {@UniqueConstraint(columnNames={"organization_id"})}) 
public class Organization implements Serializable 
{ 

    … 

    @OneToMany(fetch = FetchType.EAGER, cascade = {CascadeType.PERSIST, CascadeType.MERGE}) 
    @JoinTable(name = "cb_organization_address", 
       joinColumns = @JoinColumn(name = "organization_id"), 
       inverseJoinColumns = @JoinColumn(name = "address_id")) 
    /* List of addresses associated with this organization */ 
    private List<Address> addresses; 

請注意連接表「cb_organization_address」。它的主鍵是一個varchar（32）非空字段，我希望成爲一個GUID，類似於組織實體的ID。如何自動生成此表的ID？當我嘗試使用地址來保存組織時，我會得到例外...

javax.persistence.PersistenceException: org.hibernate.exception.GenericJDBCException: Field 'ID' doesn't have a default value 
    org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1361) 
    org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1289) 
    org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1295) 
    org.hibernate.ejb.AbstractEntityManagerImpl.flush(AbstractEntityManagerImpl.java:976) 
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
    java.lang.reflect.Method.invoke(Method.java:597) 
    org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:240) 
    $Proxy47.flush(Unknown Source) 
    org.mainco.subco.organization.repo.OrganizationDaoImpl.save(OrganizationDaoImpl.java:63) 
    org.mainco.subco.organization.service.OrganizationServiceImpl.save(OrganizationServiceImpl.java:64) 
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
    java.lang.reflect.Method.invoke(Method.java:597) 
    org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:318) 
    org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:183) 
    org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150) 
    org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:110) 
    org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172) 
    org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202) 
    $Proxy62.save(Unknown Source) 
    org.mainco.subco.sbadmin.controllers.OrganizationController.save(OrganizationController.java:204) 
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
    java.lang.reflect.Method.invoke(Method.java:597) 
    org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:213) 
    org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:126) 
     org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:96) 
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:617) 
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:578) 
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80) 
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923) 
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852) 
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882) 
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:789) 
    javax.servlet.http.HttpServlet.service(HttpServlet.java:754) 
    javax.servlet.http.HttpServlet.service(HttpServlet.java:847) 

Answer 1

您不應該自動生成連接表的ID。相反，您應該從cb_organization_address中刪除ID列。

聯接表通常不具有自己的ID，它們僅表示兩個事物之間的關聯。因此，關係本身的「身份」往往是兩件事情相關的複合關鍵。對於你的用例，我會說這是正確的數據模型：每個組織可以有任何地址零次或一次，每個地址可以在任何數量的組織中。