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0

Looking for a solution to map multiple student_id rows under the same class_id, here is the table which has 3 fields:

integer id PRIMARY KEY
integer class_id;
integer student_id;

The table can have multiple rows under the same class_id like this:

Table = 'class'
id class_id student_id 
1, 1, 1
2, 1, 2
3, 1, 3
4, 2, 1
5, 3, 1

I'm trying to Map this into a Pojo so I can use it as follow:

List<UniClass> uniClasses = session.createQuery("FROM ClassOfStudent", ClassOfStudent.class).getResultList();

I'm trying to solve this problem by using @ElementCollection

Here is my attempt:

@Entity
@Table(name = "class")
public class ClassOfStudents
 @Id
 @Column("id")
 private long m_id;

 @Column(name = "class_id")
 private long m_classId;

 @ElementCollection
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name="class_id"))
 @Column(name = "student_id")
 private List<Long> m_studentIds;

 ....

Any idea how to make this work?

hibernate hibernate-mapping

share|improve this question

edited Mar 25 at 13:59

Shvalb

asked Mar 22 at 18:30

ShvalbShvalb

6121439

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this table describes Many-to-Many relations: between what and what? I can't use @OneToMany since it's a single entity: A one-to-many between what and what? I'm trying to experiment with @ElementCollection: a collection of what? Stored where?
  
  – JB Nizet
  Mar 22 at 18:35
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  collection of 'std_ids' stored at that table I described.
  
  – Shvalb
  Mar 22 at 18:37
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Each row of that table has one and only one std_id, stored in the column std_id of that same table. So there is no association at all, even less a collection. All you need is a column of type long.
  
  – JB Nizet
  Mar 22 at 18:39
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  there are multiple rows with uni_id=1 and std_id = 1,2,3 - please look at the example.
  
  – Shvalb
  Mar 22 at 18:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So what? There are several persons with the name "John". That doesn't mean that a person, identified by a unique person_id, has several names. A person has just one name. Just like a uni_class identified by an id has just one std_id.
  
  – JB Nizet
  Mar 22 at 18:42
  

  
  
  
  

 | 
show 4 more comments

0

Looking for a solution to map multiple student_id rows under the same class_id, here is the table which has 3 fields:

integer id PRIMARY KEY
integer class_id;
integer student_id;

The table can have multiple rows under the same class_id like this:

Table = 'class'
id class_id student_id 
1, 1, 1
2, 1, 2
3, 1, 3
4, 2, 1
5, 3, 1

I'm trying to Map this into a Pojo so I can use it as follow:

List<UniClass> uniClasses = session.createQuery("FROM ClassOfStudent", ClassOfStudent.class).getResultList();

I'm trying to solve this problem by using @ElementCollection

Here is my attempt:

@Entity
@Table(name = "class")
public class ClassOfStudents
 @Id
 @Column("id")
 private long m_id;

 @Column(name = "class_id")
 private long m_classId;

 @ElementCollection
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name="class_id"))
 @Column(name = "student_id")
 private List<Long> m_studentIds;

 ....

Any idea how to make this work?

hibernate hibernate-mapping

share|improve this question

edited Mar 25 at 13:59

Shvalb

asked Mar 22 at 18:30

ShvalbShvalb

6121439

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this table describes Many-to-Many relations: between what and what? I can't use @OneToMany since it's a single entity: A one-to-many between what and what? I'm trying to experiment with @ElementCollection: a collection of what? Stored where?
  
  – JB Nizet
  Mar 22 at 18:35
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  collection of 'std_ids' stored at that table I described.
  
  – Shvalb
  Mar 22 at 18:37
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Each row of that table has one and only one std_id, stored in the column std_id of that same table. So there is no association at all, even less a collection. All you need is a column of type long.
  
  – JB Nizet
  Mar 22 at 18:39
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  there are multiple rows with uni_id=1 and std_id = 1,2,3 - please look at the example.
  
  – Shvalb
  Mar 22 at 18:40
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So what? There are several persons with the name "John". That doesn't mean that a person, identified by a unique person_id, has several names. A person has just one name. Just like a uni_class identified by an id has just one std_id.
  
  – JB Nizet
  Mar 22 at 18:42
  

  
  
  
  

 | 
show 4 more comments

Looking for a solution to map multiple student_id rows under the same class_id, here is the table which has 3 fields:

integer id PRIMARY KEY
integer class_id;
integer student_id;

The table can have multiple rows under the same class_id like this:

Table = 'class'
id class_id student_id 
1, 1, 1
2, 1, 2
3, 1, 3
4, 2, 1
5, 3, 1

I'm trying to Map this into a Pojo so I can use it as follow:

List<UniClass> uniClasses = session.createQuery("FROM ClassOfStudent", ClassOfStudent.class).getResultList();

I'm trying to solve this problem by using @ElementCollection

Here is my attempt:

@Entity
@Table(name = "class")
public class ClassOfStudents
 @Id
 @Column("id")
 private long m_id;

 @Column(name = "class_id")
 private long m_classId;

 @ElementCollection
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name="class_id"))
 @Column(name = "student_id")
 private List<Long> m_studentIds;

 ....

Any idea how to make this work?

hibernate hibernate-mapping

share|improve this question

edited Mar 25 at 13:59

Shvalb

asked Mar 22 at 18:30

ShvalbShvalb

6121439

integer id PRIMARY KEY
integer class_id;
integer student_id;

Table = 'class'
id class_id student_id 
1, 1, 1
2, 1, 2
3, 1, 3
4, 2, 1
5, 3, 1

List<UniClass> uniClasses = session.createQuery("FROM ClassOfStudent", ClassOfStudent.class).getResultList();

@Entity
@Table(name = "class")
public class ClassOfStudents
 @Id
 @Column("id")
 private long m_id;

 @Column(name = "class_id")
 private long m_classId;

 @ElementCollection
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name="class_id"))
 @Column(name = "student_id")
 private List<Long> m_studentIds;

 ....

share|improve this question

edited Mar 25 at 13:59

Shvalb

asked Mar 22 at 18:30

ShvalbShvalb

6121439

share|improve this question

edited Mar 25 at 13:59

Shvalb

asked Mar 22 at 18:30

ShvalbShvalb

6121439

asked Mar 22 at 18:30

ShvalbShvalb

6121439

asked Mar 22 at 18:30

ShvalbShvalb

6121439

1 Answer
1

active

oldest

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0

The trick that made this possible is that eventually I mapped the @Id of the class to be m_classId although it's not really the primary key of the table in the DB.

@Entity
@Table(name = "class")
public class ClassOfStudents implements Serializable

 @Id
 @Column(name = "class_id")
 private long classId;

 @ElementCollection(targetClass = Long.class, fetch = FetchType.EAGER)
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name = "class_id"))
 @Column(name = "student_id", nullable = false) 
 private List<Long> m_studentIds;
...

share|improve this answer

answered Mar 25 at 20:04

ShvalbShvalb

6121439

add a comment | 

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0

The trick that made this possible is that eventually I mapped the @Id of the class to be m_classId although it's not really the primary key of the table in the DB.

@Entity
@Table(name = "class")
public class ClassOfStudents implements Serializable

 @Id
 @Column(name = "class_id")
 private long classId;

 @ElementCollection(targetClass = Long.class, fetch = FetchType.EAGER)
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name = "class_id"))
 @Column(name = "student_id", nullable = false) 
 private List<Long> m_studentIds;
...

share|improve this answer

answered Mar 25 at 20:04

ShvalbShvalb

6121439

add a comment | 

0

The trick that made this possible is that eventually I mapped the @Id of the class to be m_classId although it's not really the primary key of the table in the DB.

@Entity
@Table(name = "class")
public class ClassOfStudents implements Serializable

 @Id
 @Column(name = "class_id")
 private long classId;

 @ElementCollection(targetClass = Long.class, fetch = FetchType.EAGER)
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name = "class_id"))
 @Column(name = "student_id", nullable = false) 
 private List<Long> m_studentIds;
...

share|improve this answer

answered Mar 25 at 20:04

ShvalbShvalb

6121439

add a comment | 

The trick that made this possible is that eventually I mapped the @Id of the class to be m_classId although it's not really the primary key of the table in the DB.

@Entity
@Table(name = "class")
public class ClassOfStudents implements Serializable

 @Id
 @Column(name = "class_id")
 private long classId;

 @ElementCollection(targetClass = Long.class, fetch = FetchType.EAGER)
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name = "class_id"))
 @Column(name = "student_id", nullable = false) 
 private List<Long> m_studentIds;
...

share|improve this answer

answered Mar 25 at 20:04

ShvalbShvalb

6121439

@Entity
@Table(name = "class")
public class ClassOfStudents implements Serializable

 @Id
 @Column(name = "class_id")
 private long classId;

 @ElementCollection(targetClass = Long.class, fetch = FetchType.EAGER)
 @CollectionTable(name = "class", joinColumns = @JoinColumn(name = "class_id"))
 @Column(name = "student_id", nullable = false) 
 private List<Long> m_studentIds;
...

share|improve this answer

answered Mar 25 at 20:04

ShvalbShvalb

6121439

answered Mar 25 at 20:04

ShvalbShvalb

6121439

answered Mar 25 at 20:04

ShvalbShvalb

6121439