如何在hibernate HQL中查询一对多的连接

Question

我尝试查询连接拖表客户和主订单表。当我总是查询错误

-Customer.java

package com.mdr.model;

import java.util.List;
import java.util.Set;

import javax.persistence.CascadeType;
//import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;
/**
 * 
 * @author Sothon
 *
 */
@Entity
@Table(name = "customer")
public class Customer{


    @Id
    @GeneratedValue
    @Column(name="id")
    private long id;
    @Column(name="name")
    private String name;
    @Column(name="person")
    private String person;
    @Column(name="mail")
    private String mail;

    private int packFlg;

    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getPerson() {
        return person;
    }
    public void setPerson(String person) {
        this.person = person;
    }
    public String getMail() {
        return mail;
    }
    public void setMail(String mail) {
        this.mail = mail;
    }


    @OneToMany
     @JoinColumn(name="customerId",referencedColumnName="customer_id")
    private List<MainOrder> mainOrder;
    public List<MainOrder> getMaiOrder() {
        return mainOrder;
    }

    public void setMaiOrder(List<MainOrder> mainOrder) {
        this.mainOrder = mainOrder;
    }


}

-MainOrder.java

package com.mdr.model;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name="main_order")
public class MainOrder{

    @Id
    @GeneratedValue
    @Column(name="id")
    private long id;
    @Column(name="file_dir")
    private String fileDir;
    @Column(name="date")
    private String date;
    @Column(name="customer_id")
    private long customerId;
    @Column(name="quantity")



    /** getter and setter */
    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getFileDir() {
        return fileDir;
    }
    public void setFileDir(String fileDir) {
        this.fileDir = fileDir;
    }
    public String getDate() {
        return date;
    }
    public void setDate(String date) {
        this.date = date;
    }
    public long getCustomerId() {
        return customerId;
    }
    public void setCustomerId(long customerId) {
        this.customerId = customerId;
    }

       @ManyToOne
    private Customer customer;
       public Customer getCustomer() {
         return customer;
     }

     public void setCustomer(Customer customer) {
            this.customer = customer;
     }

}

我想要来自表客户的查询电子邮件。我尝试这样的查询

public String getCustomerEmail(){
        Session session = HibernateSessionFactory.getSession();
        //Session session = sessionFactory.openSession();
        String maxOrderNoDay = "";
        try{
            maxOrderNoDay = (String) session.createQuery("select mail from Customer cus join cus.MainOrder man WHERE man.id = 1").uniqueResult();

        }catch(HibernateException e){
            e.printStackTrace();
        }finally{
            session.close();
            return  maxOrderNoDay;
        }
    }

它总是显示像这样的错误

    java.lang.NullPointerException
    at org.hibernate.cfg.annotations.CollectionBinder.bindCollectionSecondPass(CollectionBinder.java:1456)
    at org.hibernate.cfg.annotations.CollectionBinder.bindOneToManySecondPass(CollectionBinder.java:864)
    at org.hibernate.cfg.annotations.CollectionBinder.bindStarToManySecondPass(CollectionBinder.java:779)
    at org.hibernate.cfg.annotations.CollectionBinder$1.secondPass(CollectionBinder.java:728)
    at org.hibernate.cfg.CollectionSecondPass.doSecondPass(CollectionSecondPass.java:70)
    at org.hibernate.cfg.Configuration.originalSecondPassCompile(Configuration.java:1695)
    at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1424)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1844)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1928)
    at com.mdr.dao.HibernateSessionFactory.<clinit>(HibernateSessionFactory.java:32)
    at com.mdr.dao.MainOrderDaoImpl.getCustomerEmail(MainOrderDaoImpl.java:104)
    at com.mdr.dao.TestDao.main(TestDao.java:138)
%%%% Error Creating SessionFactory %%%%
org.hibernate.DuplicateMappingException: Duplicate collection role mapping com.mdr.model.Customer.mainOrder
    at org.hibernate.cfg.Configuration$MappingsImpl.addCollection(Configuration.java:2859)
    at org.hibernate.cfg.annotations.CollectionBinder.bind(CollectionBinder.java:527)
    at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:2156)
    at org.hibernate.cfg.AnnotationBinder.processIdPropertiesIfNotAlready(AnnotationBinder.java:963)
    at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:796)
    at org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:3788)
    at org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3742)
    at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1410)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1844)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1928)
    at com.mdr.dao.HibernateSessionFactory.rebuildSessionFactory(HibernateSessionFactory.java:71)
    at com.mdr.dao.HibernateSessionFactory.getSession(HibernateSessionFactory.java:54)
    at com.mdr.dao.MainOrderDaoImpl.getCustomerEmail(MainOrderDaoImpl.java:104)
    at com.mdr.dao.TestDao.main(TestDao.java:138)
Exception in thread "main" java.lang.NullPointerException
    at com.mdr.dao.MainOrderDaoImpl.getCustomerEmail(MainOrderDaoImpl.java:113)
    at com.mdr.dao.TestDao.main(TestDao.java:138)

-hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>
        <property name="javax.persistence.validation.mode">none</property>
        <!-- Database connection settings -->

        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>

        <property name="connection.url">jdbc:mysql://localhost:3306/mydb</property>
        <property name="connection.username">root</property>
        <property name="connection.password"></property>

        <property name="c3p0.min_size">5</property>
        <property name="c3p0.max_size">20</property>
        <property name="c3p0.timeout">1800</property>
        <property name="c3p0.max_statements">50</property>
<!--        

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLInnoDBDialect</property>
        <!-- Enable Hibernate's current session context -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">update</property>

        <property name="hibernate.connection.characterEncoding">UTF-8</property>



        <!-- Mapping (Map the Bean Class to Table in Database) -->
        <mapping class="com.mdr.model.Customer" />
        <mapping class="com.mdr.model.MainOrder" />
</session-factory>
</hibernate-configuration>

请帮助我！

Answer 1

在customer.java

中尝试使用此功能

@Entity
@Table(name = "customer")
public class Customer {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private long id;
    @Column(name = "name")
    private String name;
    @Column(name = "person")
    private String person;
    @Column(name = "mail")
    private String mail;
    private int packFlg;

    @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL)
    private List<MainOrder> mainOrder;

    // generate getter and setter here
}

班级MainOrder.java

@Entity
@Table(name = "main_order")
public class MainOrder {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private long id;
    @Column(name = "file_dir")
    private String fileDir;
    @Column(name = "date")
    private String date;

    @ManyToOne
    @JoinColumn(name = "customer_id", nullable = false)
    private Customer customer;

    // generate getter and setter here 
}

您收到电子邮件的方法，

public String getCustomerEmail(){
        Session session = HibernateSessionFactory.getSession();
        //Session session = sessionFactory.openSession();
        String maxOrderNoDay = "";
        try{
            maxOrderNoDay = (String) session.createQuery("select cus.mail from Customer cus join cus.mainOrder man WHERE man.id = 1").uniqueResult();
            //
            System.out.println(maxOrderNoDay);

        }catch(HibernateException e){
            e.printStackTrace();
        }finally{
            session.close();
            return  maxOrderNoDay;
        }
    }

要打印电子邮件地址，您也可以使用条件查询，有关详细参考，请访问criteria-query-example

建议：请使用正确的命名约定。

Answer 2

映射文件中有错误

 org.hibernate.DuplicateMappingException: Duplicate class/entity mapping com.mdr.model.EmployeeTest

检查你的地图文件，检查HEAD request（可能这可能有帮助）

Answer 3

如果将joincolumn批注添加到MainOrder类

，该怎么办？

@ManyToOne
@JoinColumn(name = "customer_id", referencedColumnName = "id")
private Customer customer;

并删除

@Column(name="customer_id")
private long customerId;