java - JPA 和 Hibernate - id 生成器

Question

我正在使用 JPA 和 Hibernate 框架，当我尝试插入新记录时，hibernate 返回错误，因为列“EMPLOYEE_ID”为空。类(class)员工:

@Entity
public class Employee implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true, mappedBy="employee")
    private List<Shift> shifts = new ArrayList<Shift>();

类别转换:

@Entity
public class Shift implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.EAGER, cascade = CascadeType.DETACH)
    @JoinColumn(name="EMPLOYEE_ID")
    private Employee employee;

插入记录的那一刻:

 Employee employee = getEmployeeData();
 entityManager.persist(employee);
 entityManager.flush();

获取员工数据方法:

private Employee getEmployeeData() {
    Employee employee = new Employee();
    Shift shift = new Shift();
    employee.addShift(shift);
    return employee;
}

异常(exception):

9:24:35,730 ERROR [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (default task-56) cannot insert the value NULL into 'EMPLOYEE_ID' column, table 'DB.dbo.SHIFT'; the column does not allow nulls. Failure to INSERT
09:24:35,750 ERROR [org.jboss.as.ejb3.invocation] (default task-56) JBAS014134: EJB Invocation failed on component PersistenceScopeExtendedEJB for method public abstract void objective.commons.jpa.threadedpersistence.scope.PersistenceScopeWithTransaction.save(): javax.ejb.EJBException: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
    at org.jboss.as.ejb3.tx.CMTTxInterceptor.handleExceptionInOurTx(CMTTxInterceptor.java:190) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.as.ejb3.tx.CMTTxInterceptor.invokeInOurTx(CMTTxInterceptor.java:275) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.as.ejb3.tx.CMTTxInterceptor.required(CMTTxInterceptor.java:340) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.as.ejb3.tx.CMTTxInterceptor.processInvocation(CMTTxInterceptor.java:239) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ejb3.component.interceptors.CurrentInvocationContextInterceptor.processInvocation(CurrentInvocationContextInterceptor.java:41) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ejb3.component.invocationmetrics.WaitTimeInterceptor.processInvocation(WaitTimeInterceptor.java:43) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ejb3.security.SecurityContextInterceptor.processInvocation(SecurityContextInterceptor.java:95) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ejb3.component.interceptors.ShutDownInterceptorFactory$1.processInvocation(ShutDownInterceptorFactory.java:64) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ejb3.component.interceptors.LoggingInterceptor.processInvocation(LoggingInterceptor.java:59) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ee.component.NamespaceContextInterceptor.processInvocation(NamespaceContextInterceptor.java:50)
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.as.ejb3.component.interceptors.AdditionalSetupInterceptor.processInvocation(AdditionalSetupInterceptor.java:55) [wildfly-ejb3-8.1.0.Final.jar:8.1.0.Final]
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.invocation.ContextClassLoaderInterceptor.processInvocation(ContextClassLoaderInterceptor.java:64)
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.invocation.InterceptorContext.run(InterceptorContext.java:326)
    at org.wildfly.security.manager.WildFlySecurityManager.doChecked(WildFlySecurityManager.java:448)
    at org.jboss.invocation.AccessCheckingInterceptor.processInvocation(AccessCheckingInterceptor.java:61)
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.invocation.InterceptorContext.run(InterceptorContext.java:326)
    at org.jboss.invocation.PrivilegedWithCombinerInterceptor.processInvocation(PrivilegedWithCombinerInterceptor.java:80)
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61)
    at org.jboss.as.ee.component.ViewService$View.invoke(ViewService.java:185)
    at org.jboss.as.ee.component.ViewDescription$1.processInvocation(ViewDescription.java:182)
    at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:309)
    at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61)
    at org.jboss.as.ee.component.ProxyInvocationHandler.invoke(ProxyInvocationHandler.java:73)
    at objective.commons.jpa.threadedpersistence.scope.PersistenceScopeWithTransaction$$$view3.save(Unknown Source) [commons-jpa-threadedpersistence-1.0.4-SNAPSHOT.jar:]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.8.0_40]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) [rt.jar:1.8.0_40]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.8.0_40]
    at java.lang.reflect.Method.invoke(Method.java:497) [rt.jar:1.8.0_40]
    at org.jboss.weld.util.reflection.Reflections.invokeAndUnwrap(Reflections.java:401) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.bean.proxy.EnterpriseBeanProxyMethodHandler.invoke(EnterpriseBeanProxyMethodHandler.java:99) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.bean.proxy.EnterpriseTargetBeanInstance.invoke(EnterpriseTargetBeanInstance.java:56) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.bean.proxy.InjectionPointPropagatingEnterpriseTargetBeanInstance.invoke(InjectionPointPropagatingEnterpriseTargetBeanInstance.java:65) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.bean.proxy.ProxyMethodHandler.invoke(ProxyMethodHandler.java:100) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at objective.commons.jpa.threadedpersistence.extended.scope.PersistenceScopeWithTransaction$2061512995$Proxy$_$$_Weld$EnterpriseProxy$.save(Unknown Source) [commons-jpa-threadedpersistence-extended-1.0.4-SNAPSHOT.jar:]
    at pontualpro.funcionario.CadastroFuncionarioController.salvar(CadastroFuncionarioController.java:190) [classes:]
    at pontualpro.funcionario.CadastroFuncionarioController$Proxy$_$$_WeldSubclass.salvar(Unknown Source) [classes:]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.8.0_40]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) [rt.jar:1.8.0_40]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.8.0_40]
    at java.lang.reflect.Method.invoke(Method.java:497) [rt.jar:1.8.0_40]
    at org.jboss.weld.interceptor.proxy.SimpleInterceptionChain.interceptorChainCompleted(SimpleInterceptionChain.java:52) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.interceptor.chain.AbstractInterceptionChain.invokeNextInterceptor(AbstractInterceptionChain.java:83) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.interceptor.proxy.InterceptorInvocationContext.proceed(InterceptorInvocationContext.java:146) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at objective.commons.jpa.threadedpersistence.cdi.AutomaticPersistenceScopeInterceptor.manageScope(AutomaticPersistenceScopeInterceptor.java:30) [commons-jpa-threadedpersistence-1.0.4-SNAPSHOT.jar:]
    at sun.reflect.GeneratedMethodAccessor57.invoke(Unknown Source) [:1.8.0_40]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.8.0_40]
    at java.lang.reflect.Method.invoke(Method.java:497) [rt.jar:1.8.0_40]
    at org.jboss.weld.interceptor.proxy.SimpleMethodInvocation.invoke(SimpleMethodInvocation.java:30) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.interceptor.chain.AbstractInterceptionChain.invokeNext(AbstractInterceptionChain.java:103) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.interceptor.chain.AbstractInterceptionChain.invokeNextInterceptor(AbstractInterceptionChain.java:81) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.interceptor.proxy.InterceptorMethodHandler.executeInterception(InterceptorMethodHandler.java:48) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.interceptor.proxy.InterceptorMethodHandler.invoke(InterceptorMethodHandler.java:41) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.bean.proxy.CombinedInterceptorAndDecoratorStackMethodHandler.invoke(CombinedInterceptorAndDecoratorStackMethodHandler.java:53) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at pontualpro.funcionario.CadastroFuncionarioController$Proxy$_$$_WeldSubclass.salvar(Unknown Source) [classes:]
    at pontualpro.funcionario.CadastroFuncionarioController$Proxy$_$$_WeldClientProxy.salvar(Unknown Source) [classes:]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.8.0_40]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) [rt.jar:1.8.0_40]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.8.0_40]
    at java.lang.reflect.Method.invoke(Method.java:497) [rt.jar:1.8.0_40]
    at com.sun.el.parser.AstValue.invoke(AstValue.java:275) [javax.el-3.0.0.jar:]
    at com.sun.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:304) [javax.el-3.0.0.jar:]
    at org.jboss.weld.util.el.ForwardingMethodExpression.invoke(ForwardingMethodExpression.java:40) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.el.WeldMethodExpression.invoke(WeldMethodExpression.java:50) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.util.el.ForwardingMethodExpression.invoke(ForwardingMethodExpression.java:40) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at org.jboss.weld.el.WeldMethodExpression.invoke(WeldMethodExpression.java:50) [weld-core-impl-2.1.2.Final.jar:2014-01-09 09:23]
    at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:105) [jsf-impl-2.2.6-jbossorg-4.jar:]
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:87) [jboss-jsf-api_2.2_spec-2.2.6.jar:2.2.6]
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102) [jsf-impl-2.2.6-jbossorg-4.jar:]
    at javax.faces.component.UICommand.broadcast(UICommand.java:315) [jboss-jsf-api_2.2_spec-2.2.6.jar:2.2.6]
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:790) [jboss-jsf-api_2.2_spec-2.2.6.jar:2.2.6]
    at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:1282) [jboss-jsf-api_2.2_spec-2.2.6.jar:2.2.6]
    at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:81) [jsf-impl-2.2.6-jbossorg-4.jar:]
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101) [jsf-impl-2.2.6-jbossorg-4.jar:]
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:198) [jsf-impl-2.2.6-jbossorg-4.jar:]
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:646) [jboss-jsf-api_2.2_spec-2.2.6.jar:2.2.6]
    at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:85) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:130) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at org.ocpsoft.rewrite.servlet.RewriteFilter.doFilter(RewriteFilter.java:205) [rewrite-servlet-2.0.12.Final.jar:2.0.12.Final]
    at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:132) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:85) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:61) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:229) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletInitialHandler.dispatchToPath(ServletInitialHandler.java:172) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.spec.RequestDispatcherImpl.forward(RequestDispatcherImpl.java:159) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at org.ocpsoft.rewrite.servlet.impl.HttpRewriteResultHandler.handleResult(HttpRewriteResultHandler.java:41) [rewrite-servlet-2.0.12.Final.jar:2.0.12.Final]
    at org.ocpsoft.rewrite.servlet.RewriteFilter.rewrite(RewriteFilter.java:268) [rewrite-servlet-2.0.12.Final.jar:2.0.12.Final]
    at org.ocpsoft.rewrite.servlet.RewriteFilter.doFilter(RewriteFilter.java:188) [rewrite-servlet-2.0.12.Final.jar:2.0.12.Final]
    at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:132) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at pontualpro.security.SegurancaFilter.doFilter(SegurancaFilter.java:43) [classes:]
    at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:132) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at pontualpro.commons.SetEncodingFilter.doFilter(SetEncodingFilter.java:19) [classes:]
    at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:60) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:132) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:85) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:61) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at org.wildfly.extension.undertow.security.SecurityContextAssociationHandler.handleRequest(SecurityContextAssociationHandler.java:78)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:113) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.security.ServletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:56) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:45) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:61) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:58) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:70) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.security.handlers.SecurityInitialHandler.handleRequest(SecurityInitialHandler.java:76) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at org.wildfly.extension.undertow.security.jacc.JACCContextIdHandler.handleRequest(JACCContextIdHandler.java:61)
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:25) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:240) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:227) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:73) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:146) [undertow-servlet-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.Connectors.executeRootHandler(Connectors.java:177) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:727) [undertow-core-1.0.15.Final.jar:1.0.15.Final]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [rt.jar:1.8.0_40]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [rt.jar:1.8.0_40]
    at java.lang.Thread.run(Thread.java:745) [rt.jar:1.8.0_40]

我已经知道 Employee 表的 id 仅在提交时生成，但是如何修复它以在 Employee 类中保留“List shift”？

Answer 1

您的实体之间具有双向一对多关联，因此您必须通过添加行来在 Java 代码中保持相同的关联 --shift.setEmployee(员工);

因此您的 getEmployeeData 代码将如下所示:

private Employee getEmployeeData() {
    Employee employee = new Employee();
    Shift shift = new Shift();
    employee.addShift(shift);
    shift.setEmployee(employee); // Adding this line fixes the issue.
    return employee;
}