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一尘不染

如何使HIbernate提取根实体的所有属性以及仅关联实体的特定属性？

hibernate

我有根实体Hostel及其单一关联User owner。

当我获取Hostel实体时，我需要热切地获取User owner，但是只有它owner的3个属性：userId，firstName，lastName。

现在，我的条件查询是：

Criteria criteria = currenSession().createCriteria(Hostel.class);

criteria.add(Restrictions.ge("endDate", Calendar.getInstance()));
if (StringUtils.notNullAndEmpty(country)) {
        criteria.add(Restrictions.eq("country", country));
}

Long count = (Long) criteria
            .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
            .setProjection(Projections.rowCount()).uniqueResult();

criteria.setFetchMode("owner", FetchMode.SELECT);
criteria.addOrder(Order.desc("rating"));

// needed to reset previous rowCount projection
criteria.setProjection(null);

// retrieve owner association
criteria.createAlias("owner", "owner", JoinType.LEFT_OUTER_JOIN)
        .setProjection(
                Projections.projectionList()
                        .add(Projections.property("owner.userId"))
                        .add(Projections.property("owner.firstName"))
                        .add(Projections.property("owner.lastName")));

criteria.setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP);

接下来，我criteria.list()得到了sql语句，该语句仅选择owner投影列表中指定的3个属性。但是它不会选择根Hostel实体的任何属性。生成的查询是：

select
    owner1_.user_id as y0_,
    owner1_.firstName as y1_,
    owner1_.lastName as y2_ 
from
    HOSTEL this_ 
left outer join
    USER owner1_ 
        on this_.owner_fk=owner1_.user_id 
where
    this_.end_date>=? 
    and this_.country=?        
order by
    this_.rating desc limit ?

该查询不起作用，因为它返回了五个Map为空的。五个映射是因为有五行Hostel匹配where条件。我创建了简单的sql查询，它工作正常，因此问题仅在这里。

如何强制hibernate获取根Hostel实体的所有属性以及关联User owner实体的仅3个属性？

*我尝试使用 *EDIT
，getSessionFactory().getClassMetadata(Hostel.class)但它给出了有关在中映射枚举的错误Hostel。因此，我后退以Hostel手动列出属性。现在，我的条件查询是：

// retrieve owner association
        criteria.createAlias("owner", "owner", JoinType.LEFT_OUTER_JOIN);
        criteria.setProjection(Projections.projectionList()
                .add(Projections.property("hostelId"))
                .add(Projections.property("address"))
                .add(Projections.property("country"))
                .add(Projections.property("region"))
                .add(Projections.property("gender"))
                .add(Projections.property("owner.userId"))
                .add(Projections.property("owner.firstName"))
                .add(Projections.property("owner.lastName")));

List<Hostel> hostels = criteria.list();

for (Hostel hostel : hostels) { // at this line I get error java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.home.hostme.entity.Hostel
            User owner = hostel.getOwner();
            System.out.println("owner=" + owner);
        }

请注意，我删除了ALIAS_TO_ENTITY_MAP结果转换器。这产生了这样的mysql查询：

select
    this_.hostel_id as y0_,
    this_.address as y1_,
    this_.country as y2_,
    this_.region as y3_,
    this_.gender as y4_,
    owner1_.user_id as y5_,
    owner1_.firstName as y6_,
    owner1_.lastName as y7_ 
from
    HOSTEL this_ 
left outer join
    USER owner1_ 
        on this_.owner_fk=owner1_.user_id 
where
    this_.end_date>=? 
    and this_.country=? 
order by
    this_.rating desc limit ?

在for-each循环中得到这样的错误：

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.home.hostme.entity.Hostel
    at com.home.hostme.dao.impl.HostelDaoImpl.findHostelBy(HostelDaoImpl.java:168)
    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.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:183)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:155)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
    at com.sun.proxy.$Proxy64.findHostelBy(Unknown Source)
    at com.home.hostme.service.HostelService.findHostelBy(HostelService.java:27)
    at com.home.hostme.service.HostelService$$FastClassByCGLIB$$74db5b21.invoke(<generated>)
    at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
    at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:698)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:96)
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:260)
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:94)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:631)
    at com.home.hostme.service.HostelService$$EnhancerByCGLIB$$7af3bc10.findHostelBy(<generated>)
    at com.home.hostme.web.hostel.HostelController.doSearch(HostelController.java:94)
    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.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:219)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:745)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:686)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:925)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:856)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:936)
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:838)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:812)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

这个错误表示我没有Hostel在结果列表中输入文字hostels。我什至尝试用以下方法在结果列表“旅馆”中找出元素的类别：

List hostels = criteria.list();
        System.out.println("firstRow.class=" + hostels.get(0).getClass());

它打印：

firstRow.class=class [Ljava.lang.Object;

然后，我尝试设置ALIAS_TO_ENTITY_MAP新的ProjectionList，但结果列表“旅馆”为：

[{}, {}, {}, {}, {}]

五张空的地图。五，因为db（table hostel）中有5行与where子句匹配。

然后我完全删除投影列表和hibernate检索5个旅馆和5个相关联的User ownerS和owner的图像作为预期。

问题是如何停止hibernate检索关联的关联Image实体User owner。最好的方法是仅获取3个相关联的道具User owner。

谢谢！


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2020-06-20

共1个答案

一尘不染

您可以通过直接查询来做到这一点：

Query query = session.createQuery("SELECT hostel, owner.id, owner.firstname, "
        +"owner.lastname FROM Hostel hostel LEFT OUTER JOIN hostel.ower AS owner");
List list = query.list();

生成一个SQL，如：

选择hostel0_.id作为col_0_0_，user1_.id作为col_1_0_，user1_.firstname作为col_2_0_，user1_.lastname作为col_3_0_，hostel0_.id作为id1_0_，hostel0_.name作为name2_0_，…，hostel0_.owner_id作为user_id4_0_外部联接user1_.id
= hostel0_.owner_id上的用户user1_

包含Hostel的所有字段，以及User的所有必填字段。

用获得的名单criteria.list()是List<Object[]>其行是 [ Hostel, Integer, String, String]

您可以使用来获取某些内容Criteria，但Criteria要比查询更严格。我找不到允许混合实体和字段的任何API。据我所知，不可能获得包含实体（旅馆）和与关联（owner.userId，owner.firstName，owner.lastName）分开的字段的行。

我能想象的唯一方法是明确列出Hostels的所有字段：

criteria.createAlias("owner", "owner", JoinType.LEFT_OUTER_JOIN)
    .setProjection(
            Projections.projectionList()
                    .add(Projections.property("hostelId"))
                    .add(Projections.property("country"))
                    .add(Projections.property("endDate"))
                    ...
                    ... all other properties from Hostel
                    ...
                    .add(Projections.property("owner.userId"))
                    .add(Projections.property("owner.firstName"))
                    .add(Projections.property("owner.lastName")));

您可以通过使用元数据（不要忘记id
…）来实现一些自动化（注意：…）：我使用别名投影只是为了以后可以使用包装器类，如果直接使用标量值，则可以安全地省略的Projection.alias：

    ProjectionList hostelProj = Projections.projectionList();
    String id = sessionFactory.getClassMetadata(Hostel.class)
            .getIdentifierPropertyName();
    hostelProperties.add(Projections.alias(Projections.property(id),id));
    for (String prop: sessionFactory.getClassMetadata(Hostel.class).getPropertyNames()) {
        hostelProperties.add(Projections.alias(Projections.property(prop), prop));
    }
    Criteria criteria = session.createCriteria(Hostel.class);
    criteria.createAlias("owner", "owner", JoinType.LEFT_OUTER_JOIN);
    criteria.setProjection(
            Projections.projectionList()
                    .add(hostelProj)
                    .add(Projections.property("owner.id"))
                    .add(Projections.property("owner.firstName"))
                    .add(Projections.property("owner.lastName")));
    List list = criteria.list();

这样正确生成

从酒店this_中选择this_.id作为y0_，this_.name作为y1_，…，this_.user_id作为y3_，owner1_.id作为y4_，owner1_.firstname作为y5_，owner1_.lastname作为y6_，在this_上从外部加入用户owner1_
.user_id = owner1_.id

但是您将无法使用，criteria.setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP)因为结果集不完全是来自字段的图像Hostel（即使没有别名）。实际上，列表是List<Object[]>带有行的，其中包含所有来自的单独字段，Hostel然后是来自的3个必填字段owner。

您将需要添加一个包含a Hostel和3个其他字段的包装器类，以使用an AliasToBeanResultTransformer和获取true
Hostel对象：

public class HostelWrapper {
    private Hostel hostel;
    private int owner_id;
    private String owner_firstName;
    private String owner_lastName;

    public HostelWrapper() {
        hostel = new Hostel();
    }

    public Hostel getHostel() {
        return hostel;
    }
    public void setId(int id) {
        hostel.setId(id);
    }
    public void setOwner(User owner) {
        hostel.setOwner(owner);
    }
    // other setters for Hostel fields ...

    public int getOwner_id() {
        return owner_id;
    }
    public void setOwner_id(Integer owner_id) {
    // beware : may be null because of outer join
        this.owner_id = (owner_id == null) ? 0 : owner_id;
    }
    //getters and setters for firstName and lastName ...
}

然后您可以成功编写：

criteria.setResultTransformer(new AliasToBeanResultTransformer(HostelWrapper.class));
List<HostelWrapper> hostels = criteria.list();

Hostel hostel = hostels.get(0).getHostel();
String firstName = hostels.get(0).getFirstName();

我可以确认的是，当没有主人hostel.getOwner()是空的，当有一个，hostel.getOwner().getId()等于getOwner_id()和该访问不产生任何额外的查询。但是，任何访问的其他领域hostel.getOwner()，甚至firstName或lastName产生，因为一个User实体并没有在会议上加载。

最常见的用法应该是：

for (HostelWrapper hostelw: criteria.list()) {
    Hostel hostel = hostelw.getHostel();
    // use hostel, hostelw.getOwner_firstName and hostelw.getOwner_lastName
}
2020-06-20