title: I wonder the reason 'detached entity passed to persist' happens
body: <p>I expect that persist entity(parent) will flush automatically cause there's some changes in parent's property and I expect that children and grandson will be saved to db because of <code>cascade = CascadeType.ALL</code></p>
  
  <p>but it doesn't, and had some error detached entity passed to persist - i don't understand why those are detached entity</p>
  
  <p>I tried save again right before <code>findAll()</code>, and It works</p>
  
  <pre class="lang-java prettyprint-override"><code>
   categoryRepository.save(parent);
   log.info("출력 : '{}'", categoryRepository.findAll());
  
  </code></pre>
  
  <h2>code</h2>
  
  <ol>
  <li>domain code</li>
  </ol>
  
  <pre class="lang-java prettyprint-override"><code>@Entity
  @Getter
  @Setter
  @NoArgsConstructor
  public class MenuCategory {
  
      @Id
      @GeneratedValue(strategy = GenerationType.AUTO)
      private Long id;
  
      @Size(min = 1, max = 50)
      private String name;
  
      @ManyToOne
      @JoinColumn(name = "parentId")
      @JsonIgnore
      private MenuCategory parent;
  
      @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
      @JoinColumn(name = "parentId")
      private List&lt;MenuCategory&gt; children = new ArrayList&lt;&gt;();
  
      public MenuCategory(Long id, String name) {
          this.id = id;
          this.name = name;
      }
  
      public MenuCategory(Long id, @Size(min = 1, max = 50) String name, MenuCategory parent, List&lt;MenuCategory&gt; children) {
          this.id = id;
          this.name = name;
          this.parent = parent;
          this.children = children;
      }
  
      public MenuCategory addChild(MenuCategory child) {
          children.add(child);
          child.setParent(this);
          return this;
      }
  
      @Override
      public String toString() {
          return "MenuCategory{" +
                  "id=" + id +
                  ", name='" + name + '\'' +
                  ", children=" + children +
                  '}';
      }
  }
  
  </code></pre>
  
  <ol start="2">
  <li>testcode</li>
  </ol>
  
  <pre class="lang-java prettyprint-override"><code> @Test
      public void data_structure_test() {
          MenuCategory menuCategory = new MenuCategory(1L, "최상");
          MenuCategory parent = categoryRepository.save(menuCategory);
  
          MenuCategory secondCategory = new MenuCategory(2L, "1단계 첫번째");
          MenuCategory thirdCategory = new MenuCategory(3L, "1단계 두번째");
  
          parent.addChild(secondCategory);
          parent.addChild(thirdCategory);
  
  
          MenuCategory fourthCategory = new MenuCategory(4L, "2단계 첫번째");
          thirdCategory.addChild(fourthCategory);
  
          log.info("출력 : '{}'", categoryRepository.findAll());
  
          assertThat(categoryRepository.findById(1L).get().getChildren().size()).isEqualTo(2);
      }
  </code></pre>
  
  <h2>error</h2>
  
  <pre class="lang-java prettyprint-override"><code>2019-04-19 11:57:46.953  INFO 28426 --- [           main] o.s.t.c.transaction.TransactionContext   : Rolled back transaction for test: [DefaultTestContext@56528192 testClass = MenuCategoryRepositoryTest, testInstance = codesquad.domain.MenuCategoryRepositoryTest@460d0a57, testMethod = data_structure_test@MenuCategoryRepositoryTest, testException = org.springframework.dao.InvalidDataAccessApiUsageException: detached entity passed to persist: codesquad.domain.MenuCategory; nested exception is org.hibernate.PersistentObjectException: detached entity passed to persist: codesquad.domain.MenuCategory, mergedContextConfiguration = [MergedContextConfiguration@6e0dec4a testClass = MenuCategoryRepositoryTest, locations = '{}', classes = '{class codesquad.BaeminchanApplication}', contextInitializerClasses = '[]', activeProfiles = '{}', propertySourceLocations = '{}', propertySourceProperties = '{org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}', contextCustomizers = set[[ImportsContextCustomizer@96def03 key = [org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration, org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration, org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration, org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration, org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration, org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration, org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration, org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration, org.springframework.boot.test.autoconfigure.jdbc.TestDatabaseAutoConfiguration, org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManagerAutoConfiguration]], org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@4ae82894, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@16022d9d, org.springframework.boot.test.mock.mockito.MockitoContextCustomizer@0, org.springframework.boot.test.autoconfigure.OverrideAutoConfigurationContextCustomizerFactory$DisableAutoConfigurationContextCustomizer@2698dc7, org.springframework.boot.test.autoconfigure.filter.TypeExcludeFiltersContextCustomizer@351584c0, org.springframework.boot.test.autoconfigure.properties.PropertyMappingContextCustomizer@e580919e, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverContextCustomizerFactory$Customizer@74294adb], contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map[[empty]]]
  
  org.springframework.dao.InvalidDataAccessApiUsageException: detached entity passed to persist: codesquad.domain.MenuCategory; nested exception is org.hibernate.PersistentObjectException: detached entity passed to persist: codesquad.domain.MenuCategory
  
      at org.springframework.orm.jpa.vendor.HibernateJpaDialect.convertHibernateAccessException(HibernateJpaDialect.java:280)
      at org.springframework.orm.jpa.vendor.HibernateJpaDialect.translateExceptionIfPossible(HibernateJpaDialect.java:225)
      at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.translateExceptionIfPossible(AbstractEntityManagerFactoryBean.java:527)
      at org.springframework.dao.support.ChainedPersistenceExceptionTranslator.translateExceptionIfPossible(ChainedPersistenceExceptionTranslator.java:61)
      at org.springframework.dao.support.DataAccessUtils.translateIfNecessary(DataAccessUtils.java:242)
      at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:153)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:135)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(SurroundingTransactionDetectorMethodInterceptor.java:61)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212)
      at com.sun.proxy.$Proxy129.findAll(Unknown Source)
      at codesquad.domain.MenuCategoryRepositoryTest.data_structure_test(MenuCategoryRepositoryTest.java:43)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:498)
      at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
      at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
      at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
      at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
      at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:73)
      at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:83)
      at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
      at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
      at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
      at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
      at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
      at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
      at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
      at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
      at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
      at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
      at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
      at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
      at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
      at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
      at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
      at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
      at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
      at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
      at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
      at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
  Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: codesquad.domain.MenuCategory
      at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:124)
      at org.hibernate.internal.SessionImpl.firePersistOnFlush(SessionImpl.java:843)
      at org.hibernate.internal.SessionImpl.persistOnFlush(SessionImpl.java:836)
      at org.hibernate.engine.spi.CascadingActions$8.cascade(CascadingActions.java:341)
      at org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:467)
      at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:392)
      at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:193)
      at org.hibernate.engine.internal.Cascade.cascadeCollectionElements(Cascade.java:500)
      at org.hibernate.engine.internal.Cascade.cascadeCollection(Cascade.java:432)
      at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:395)
      at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:193)
      at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:126)
      at org.hibernate.event.internal.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:150)
      at org.hibernate.event.internal.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:141)
      at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:74)
      at org.hibernate.event.internal.DefaultAutoFlushEventListener.onAutoFlush(DefaultAutoFlushEventListener.java:44)
      at org.hibernate.internal.SessionImpl.autoFlushIfRequired(SessionImpl.java:1398)
      at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1483)
      at org.hibernate.query.internal.AbstractProducedQuery.doList(AbstractProducedQuery.java:1445)
      at org.hibernate.query.internal.AbstractProducedQuery.list(AbstractProducedQuery.java:1414)
      at org.hibernate.query.Query.getResultList(Query.java:146)
      at org.hibernate.query.criteria.internal.compile.CriteriaQueryTypeQueryAdapter.getResultList(CriteriaQueryTypeQueryAdapter.java:72)
      at org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll(SimpleJpaRepository.java:307)
      at org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll(SimpleJpaRepository.java:74)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:498)
      at org.springframework.data.repository.core.support.RepositoryComposition$RepositoryFragments.invoke(RepositoryComposition.java:377)
      at org.springframework.data.repository.core.support.RepositoryComposition.invoke(RepositoryComposition.java:200)
      at org.springframework.data.repository.core.support.RepositoryFactorySupport$ImplementationMethodExecutionInterceptor.invoke(RepositoryFactorySupport.java:629)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.doInvoke(RepositoryFactorySupport.java:593)
      at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.invoke(RepositoryFactorySupport.java:578)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:59)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:294)
      at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:185)
      at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:139)
      ... 40 more
  
  2019-04-19 11:57:46.979  INFO 28426 --- [       Thread-2] s.c.a.AnnotationConfigApplicationContext : Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@4722ef0c: startup date [Fri Apr 19 11:57:43 KST 2019]; root of context hierarchy
  2019-04-19 11:57:46.983  INFO 28426 --- [       Thread-2] j.LocalContainerEntityManagerFactoryBean : Closing JPA EntityManagerFactory for persistence unit 'default'
  </code></pre>