這個查詢有什麼問題：「SELECT b FROM Parent a JOIN a.child b」？

Question

沒有與@Any一個簡單的父/子關係：

@Entity 
public class Parent { 

    @Id 
    private String id= UUID.randomUUID().toString(); 

    @Any(metaColumn = @Column(name = "DTYPE"), fetch = FetchType.EAGER) 
     @AnyMetaDef(
      idType = "string", metaType = "string", 
      metaValues = { 
        @MetaValue(targetEntity = Child.class, value = "CHILD") 
      }) 
    @JoinColumn(name = "TYPE_ID") 
    @Cascade(CascadeType.ALL) 
    private Object child; 

    public Child getChild() { 
     return (Child)child; 
    } 

    public void setChild(Child child) { 
     this.child = child; 
    } 

    public String getId() { 
     return id; 
    } 
} 


@Entity 
public class Child { 

    @Id 
    private String id= UUID.randomUUID().toString(); 

    public String getId() { 
     return id; 
    } 
} 

每讀取調用相關的子對象可以完美運行：

Parent parent= em.find(Parent.class, parent.getId()); 
Assert.assertNotNull(parent.getChild()); 

閱讀與下面的查詢子對象失敗，並拋出ClassCastException：

Query q= em.createQuery("SELECT b FROM Parent a JOIN a.child b"); 


java.lang.ClassCastException: org.hibernate.type.AnyType cannot be cast to org.hibernate.type.ComponentType 
    at org.hibernate.hql.internal.ast.HqlSqlWalker.createFromJoinElement(HqlSqlWalker.java:389) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.joinElement(HqlSqlBaseWalker.java:3645) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.fromElement(HqlSqlBaseWalker.java:3431) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.fromElementList(HqlSqlBaseWalker.java:3309) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.fromClause(HqlSqlBaseWalker.java:706) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:562) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:299) 
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:247) 
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:249) 
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:184) 
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:137) 
    at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:105) 
    at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:80) 
    at org.hibernate.engine.query.spi.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:168) 
    at org.hibernate.internal.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:221) 
    at org.hibernate.internal.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:199) 
    at org.hibernate.internal.SessionImpl.createQuery(SessionImpl.java:1778) 
    at org.hibernate.ejb.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:291) 
    at com.hibernate.cases.AnyTest.anyTestQuery(AnyTest.java:74) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44) 
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15) 
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41) 
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20) 
    at org.junit.runners.BlockJUnit4ClassRunner.runNotIgnored(BlockJUnit4ClassRunner.java:79) 
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:71) 
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49) 
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193) 
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52) 
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191) 
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42) 
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184) 
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236) 
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50) 
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197) 

出了什麼問題？

Answer 1

一個區別是在第二個查詢中，您沒有顯式地將結果轉換爲Child.class實例。