关于hibernate和hashset的一个问题

Question

String hql = "FROM Student s JOIN s.modules m where m.moduleCode = :moduleCode"; 有一个这样的代码这个m应该是一个hashset它里面存的是一个类的对象所以我觉得这样直接点他是不对的应为这个modeCode是个属性而这个m是hashset没有这样的属性 

//课程表

@Entity
@Table(name = "MODULE")
public class Module {

    @Id
    @Column(name = "module_code")
    private String moduleCode;

    @Column(name = "module_name")
    private String moduleName;

    @ManyToMany(mappedBy = "modules")
    private Set<Student> students = new HashSet<>();

    public Module() {}

    public Module(String moduleCode, String moduleName) {
        this.moduleCode = moduleCode;
        this.moduleName = moduleName;
    }

    public String getModuleCode() {
        return moduleCode;
    }

    public String getModuleName() {
        return moduleName;
    }

    public void setModuleName(String moduleName) {
        this.moduleName = moduleName;
    }

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "Module{" +
                "moduleCode='" + moduleCode + '\'' +
                ", moduleName='" + moduleName + '\'' +
                '}';
    }
}

//学生表

package relationships.manytomany;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity
@Table(name = "STUDENT")
public class Student {

    @Id
    @Column(name = "student_id")
    private String studentId;

    @Column(name = "student_name")
    private String name;

    @Column(name = "student_age")
    private Integer age;

    @ManyToMany
    @JoinTable(
            name = "STUDENT_MODULE",
            joinColumns = {@JoinColumn(name = "student_id")},
            inverseJoinColumns = {@JoinColumn(name = "module_code")})
    private Set<Module> modules = new HashSet<>();

    public Student() {}

    public Student(String studentId, String name, Integer age) {
        this.studentId = studentId;
        this.name = name;
        this.age = age;
    }

    public String getStudentId() {
        return studentId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }


    public void setAge(Integer age) {
        this.age = age;
    }

    public Set<Module> getModules() {
        return modules;
    }

    public void setModules(Set<Module> modules) {
        this.modules = modules;
    }


    @Override
    public String toString() {
        return "Student{" +
                "studentId='" + studentId + '\'' +
                ", name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

}

//中间表

package relationships.manytomany;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import utility.HibernateUtil;

import java.util.*;

public class ManyToManyMain {

    public static void main(String[] args) {

        Session session = null;

        // create objects
        Student s1 = new Student("b1234", "Becky",19);
        Student s2 = new Student("b4321", "Jane",18);
        Student s3 = new Student("b4654", "Laura",22);
        Student s4 = new Student("b3541", "Rich",18);

        Set<Student> sl = new HashSet<>(Arrays.asList(s1,s2,s3,s4));

        Module m1 = new Module("csc1031", "Math");
        Module m2 = new Module("csc1032", "Arch");
        Module m3 = new Module("csc1033", "DB");
        Module m4 = new Module("csc1034", "P1");
        Module m5 = new Module("csc1035", "P2");

        Set<Module> ml = new HashSet<>(Arrays.asList(m1,m2,m3,m4,m5));


        // Create
        try {
            session = HibernateUtil.getSessionFactory().openSession();
            session.beginTransaction();

            // save objects to database
            for (Student stu : sl) {
                session.persist(stu);
            }
            for (Module mod : ml) {
                session.persist(mod);
            }

            // create relationship in student and save again
            // modules did not exist when first saved
            for (Student stu : sl) {
                stu.setModules(ml);
                session.persist(stu);
                
            }


            session.getTransaction().commit();

        } catch (HibernateException e) {
            if (session != null) session.getTransaction().rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }
}

//查询

package querying.relationships;

import org.hibernate.Session;
import org.hibernate.query.Query;
import relationships.manytomany.Student;
import relationships.manytomany.Module;
import utility.HibernateUtil;

import java.util.List;

public class JoinQueries {
/*
    // one-to-one should be similar to this
    public static void oneToMany(){
        Session s = HibernateUtil.getSessionFactory().openSession();

        // Find what menu each item is on
        //String hql = "SELECT i.itemName FROM ITEM i";
        String hql = "SELECT i.itemName, m.name FROM MENU_ITEM i JOIN MENU m ON i.menu.id = m.id";

        s.beginTransaction();

        // Perform query
        Query query =  s.createQuery(hql);

        // Get results
        List results = query.list();

        s.getTransaction().commit();

        // Print results
        Object[] items = results.toArray();
        for (Object item : items) {
            //Object[] parts = (Object[]) items[i];
            //for (int j = 0; j < parts.length ; j++) {
            //System.out.print(parts[j]+" ");
            //}
            System.out.println(item);
        }
    }*/

    public static void manyToManyStudentsTakingMaths(String mc){
        Session s = HibernateUtil.getSessionFactory().openSession();
        // Find all students who take CSC1031
        //s.modules=m
        String hql = "FROM Student s JOIN s.modules m where m.moduleCode = :moduleCode";

        s.beginTransaction();

        // Perform query
        Query query =  s.createQuery(hql);
        query.setParameter("moduleCode",mc);

        // Get results - returns Collection of Collection (Student, Module)
        List<Student> results = query.list();

        s.getTransaction().commit();

        // Print results
        Object[] items = results.toArray();
        for (Object item : items) {
            Object[] parts = (Object[]) item;
            Student student = (Student) parts[0];
            Module module = (Module) parts[1];
            System.out.println(student + " -> " + module);
        }
    }

 /*   public static void manyToManyStudentModule(){
        Session s = HibernateUtil.getSessionFactory().openSession();

        // Find all mappings in STUDENT_MODULE
        String hql = "FROM Student s JOIN s.modules m where m.moduleCode IN (SELECT n.moduleCode FROM Module n)";

        s.beginTransaction();

        // Perform query
        Query query =  s.createQuery(hql);

        // Get results - returns Collection of Collection (Student, Module)
        List<Student> results = query.list();

        s.getTransaction().commit();

        // Print results
        Object[] items = results.toArray();
        for (int i = 0; i <items.length ; i++) {
            Object[] parts = (Object[]) items[i];
            Student student = (Student) parts[0];
            Module module = (Module) parts[1];
            System.out.println(student + " -> " + module);
        }
    }*/

    public static void main(String[] args) {
        //oneToMany();
        manyToManyStudentsTakingMaths("csc1032");
        //manyToManyStudentModule();
    }
}

Answer 1

String hql = "FROM Student s JOIN s.modules m where m.moduleCode = :moduleCode"; 
有一个这样的代码，这个m应该是一个hashset。
它里面存的是一个类的对象，所以我觉得这样直接点它，是不对的。
因为这个modeCode是个属性，而这个m是hashset没有这样的属性

首先，要讨论问题的话。需要先把问题描述清楚。

 

SQL如果前面拼接了，“select s” 的语句话是没问题的

这个SQL翻译为原生SQL就是：

select s from student s

INNER JOIN <student 和 module的关系表> sm  ON s.id = sm.student_id

INNER JOIN module m ON m.id = sm.module_id

WHERE  m.moduleCode = :moduleCode;

 

 

m就是module的别名