🚨 Error: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing

[jaynamm]

Question 과 Answer 의 2 개의 엔티티를 조인해서 Answer 데이터가 생성할 때 question_id 를 가져와서 넣어주려고 하는데
아래와 같은 에러가 발생했다.

Question

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@Builder
@Entity
@Table(name="question")
public class Question {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false)
    private String title;

    @Column(nullable = false)
    private String author;

    @Column(nullable = false)
    private String content;

    @CreatedDate
    private LocalDateTime createdDate;

    @OneToMany(mappedBy = "question")
    private List<Answer> answerList;
}

Answer

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@Builder
@Entity
@Table(name="answer")
public class Answer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, columnDefinition = "TEXT")
    private String content;

    @Column(nullable = false)
    private String author;

    @CreatedDate
    private LocalDateTime createdDate;

    // LAZY : 실제로 요청하는 순간에 데이터를 읽어옴
    // 불필요한 요청을 막을 때 사용한다.
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "question_id")
    private Question question;
}

}

에러 코드

org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : com.encore.playground.domain.qna.entity.Answer.question -> com.encore.playground.domain.qna.entity.Question
	at org.hibernate.engine.spi.CascadingActions$8.noCascade(CascadingActions.java:383) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:169) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.event.internal.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:155) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.event.internal.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:145) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:79) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:38) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:107) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1425) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:477) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:2234) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:1930) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:439) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:183) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:281) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:101) ~[hibernate-core-6.1.7.Final.jar:6.1.7.Final]
	at org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:561) ~[spring-orm-6.0.7.jar:6.0.7]
	at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:743) ~[spring-tx-6.0.7.jar:6.0.7]
	at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:711) ~[spring-tx-6.0.7.jar:6.0.7]
	at org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.java:660) ~[spring-tx-6.0.7.jar:6.0.7]
	at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:410) ~[spring-tx-6.0.7.jar:6.0.7]
	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119) ~[spring-tx-6.0.7.jar:6.0.7]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.7.jar:6.0.7]
	at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:137) ~[spring-tx-6.0.7.jar:6.0.7]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.7.jar:6.0.7]
	at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:163) ~[spring-data-jpa-3.0.4.jar:3.0.4]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.7.jar:6.0.7]
	at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:97) ~[spring-aop-6.0.7.jar:6.0.7]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.0.7.jar:6.0.7]
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:218) ~[spring-aop-6.0.7.jar:6.0.7]
	at jdk.proxy2/jdk.proxy2.$Proxy121.save(Unknown Source) ~[na:na]
	at com.encore.playground.domain.qna.service.AnswerService.create(AnswerService.java:33) ~[classes/:na]
	at com.encore.playground.domain.qna.controller.AnswerController.createAnswer(AnswerController.java:42) ~[classes/:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:207) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:152) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:118) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1081) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:974) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1011) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:914) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:563) ~[tomcat-embed-core-10.1.7.jar:6.0]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.0.7.jar:6.0.7]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:631) ~[tomcat-embed-core-10.1.7.jar:6.0]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.springframework.security.web.FilterChainProxy.lambda$doFilterInternal$3(FilterChainProxy.java:231) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:365) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.access.intercept.AuthorizationFilter.doFilter(AuthorizationFilter.java:100) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:126) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:120) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:131) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:85) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:100) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:179) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:227) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:221) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at com.encore.playground.global.jwt.JwtAuthenticationFilter.doFilter(JwtAuthenticationFilter.java:31) ~[classes/:na]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:107) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:93) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.header.HeaderWriterFilter.doHeadersAfter(HeaderWriterFilter.java:90) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:75) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.context.SecurityContextHolderFilter.doFilter(SecurityContextHolderFilter.java:82) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.context.SecurityContextHolderFilter.doFilter(SecurityContextHolderFilter.java:69) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:62) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.session.DisableEncodeUrlFilter.doFilterInternal(DisableEncodeUrlFilter.java:42) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:233) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:191) ~[spring-security-web-6.0.2.jar:6.0.2]
	at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:352) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:268) ~[spring-web-6.0.7.jar:6.0.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.7.jar:6.0.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.7.jar:6.0.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-6.0.7.jar:6.0.7]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.7.jar:6.0.7]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:166) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:115) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:341) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:390) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:894) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1741) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-10.1.7.jar:10.1.7]
	at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

[jaynamm]

Answer

Answer 에서 해당 부분을 수정해주니까 정상적으로 작동했다.

변경 전

// LAZY : 실제로 요청하는 순간에 데이터를 읽어옴
// 불필요한 요청을 막을 때 사용한다.
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "question_id")
private Question question;

변경 후

@ManyToOne(cascade = CascadeType.PERSIST)
@JoinColumn(name = "question_id")
private Question question;

[jaynamm]

그런데 Answer 데이터가 생성되면서 참조한 question_id 의 값이 증가하고
Question 데이터가 같이 생성되는 문제가 발생했다...

[jaynamm]

AnswerService

AnswerService 에서 Answer 를 생성하는 메서드는 다음과 같다.

public List<AnswerDTO> create(AnswerDTO answerDTO, QuestionDto questionDto) {
        Answer answer = Answer.builder()
                .content(answerDTO.getContent())
                .author(answerDTO.getAuthor())
                .createdDate(LocalDateTime.now())
                .question(questionDto.toEntity())
                .build();

        answerRepository.save(answer);

        return answerList(answer.getId());
    }

이 부분에서 question 객체를 넣어서 연결시켜줘야 하는 부분이 있는데

.question(questionDto.toEntity())

이 부분에 잘 보면 Dto 에서 toEntity() 를 통해서 Question 객체를 Build 하게 된다.
결국에는 Answer 를 만드려면 Question 를 다시 생성하고 생성된 데이터를 참조해야 하는데
Question builder 가 실행되기 전에 Answer builder 가 먼저 실행하게 되서
참조해야할 대상인 Question 을 찾지 못해서 맨 위의 에러가 발생했고 Answer 데이터가 생성되지 않았던 거였다.

결론

문제의 원인은 다음과 같다.
데이터 생성의 순서가 반대로 되어 발생한 문제이다.
Answer Builder -> QuestionDto.toEntity 에서 참조할 Quesion Builder
즉, Answer 에서 Question 의 Id 값을 참조해서 가져와야 하는데 Answer 가 build 되고 난 이후에 Question 이 build 하면서
참조할 Question 의 Id 값을 참조하지 못해서 발생하는 에러였다.

따라서, 해당 부분을 어떻게 수정해야할지 생각해봐야겠다.