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 Post subject: Fetch data using single select statement in HQL in Many2One
PostPosted: Tue Dec 16, 2014 10:50 am 
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Joined: Tue Dec 16, 2014 10:47 am
Posts: 1
I have two entities student and college. A single college has multiple student.

Code:
@Entity 
public class College { 
 
    @Id 
    @GeneratedValue 
    private int collegeId; 
 
    private String collegeName; 
 
    public int getCollegeId() { 
        return collegeId; 
    } 
 
    public void setCollegeId(int collegeId) { 
        this.collegeId = collegeId; 
    } 
 
    public String getCollegeName() { 
        return collegeName; 
    } 
 
    public void setCollegeName(String collegeName) { 
        this.collegeName = collegeName; 
    } 

 
 
@Entity 
public class Student { 
 
    @Id 
    @GeneratedValue 
    private int studentId; 
 
    private String studentName; 
 
    @ManyToOne(cascade = CascadeType.ALL) 
    private College college; 
 
    public int getStudentId() { 
        return studentId; 
    } 
 
    public void setStudentId(int studentId) { 
        this.studentId = studentId; 
    } 
 
    public String getStudentName() { 
        return studentName; 
    } 
 
    public void setStudentName(String studentName) { 
        this.studentName = studentName; 
    } 
 
    public College getCollege() { 
        return college; 
    } 
 
    public void setCollege(College college) { 
        this.college = college; 
    } 


I want to fetch all the students of a particular college.

As you can see in below code the HQL query I have written is : "select student from "+Student.class.getName()+" student where student.college.collegeId = 1"

On execution of following code I two SQL queries are fired as follows:
Code:
Hibernate: select student0_.studentId as studentId1_1_, student0_.college_collegeId as college_collegeId3_1_, student0_.studentName as studentName2_1_ from mevada.Student student0_ where student0_.college_collegeId=1 
Hibernate: select college0_.collegeId as collegeId1_0_0_, college0_.collegeName as collegeName2_0_0_ from mevada.College college0_ where college0_.collegeId=? 


Ideally first query is sufficient to fetch all required students and it is working well when I fire directly from database.

why second query is executed? How can I stop Hibernate by executing this extra query?
Utility class:
Code:
public class ManyToOne { 
 
    public static void main(String[] args) { 
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("org.hibernate.examples"); 
 
        EntityManager em = emf.createEntityManager(); 
 
        College college1 = new College(); 
        college1.setCollegeName("College1"); 
 
 
        College college2 = new College(); 
        college2.setCollegeName("College2"); 
 
        Student student1 = new Student(); 
        student1.setStudentName("std1"); 
        student1.setCollege(college1); 
 
 
        Student student2 = new Student(); 
        student2.setStudentName("std2"); 
        student2.setCollege(college2); 
 
        Student student3 = new Student(); 
        student3.setStudentName("std3"); 
        student3.setCollege(college1); 
 
        Student student4 = new Student(); 
        student4.setStudentName("std4"); 
        student4.setCollege(college1); 
 
        em.getTransaction().begin(); 
 
        em.persist(college1); 
        em.persist(college2); 
        em.persist(student1); 
        em.persist(student2); 
        em.persist(student3); 
        em.persist(student4); 
 
        em.getTransaction().commit(); 
        em.close(); 
 
        em = emf.createEntityManager(); 
        em.getTransaction().begin(); 
 
        String queryString = "select student from "+Student.class.getName()+" student where student.college.collegeId = 1"; 
 
        Query query = em.createQuery(queryString); 
 
        List<Student> students = query.getResultList(); 
 
        em.getTransaction().commit(); 
        em.close(); 
        emf.close(); 
 
    } 


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