I created two entities for the tables HOBBY and STUDENT, but I have difficulties to retrieve the information I need from the Join Table.
This is the schema:
STUDENT STUDENT_HOBBY HOBBY
-------- ----------- --------
id ------------------ student_id
name hobby_id ---------- id
lastname hobby_name
--------- ---------- --------
where
Student to student_hobby = one to many
Hobby to student_hobby = one to many
(that means a student may have many hobbies and a hobby can belong to more than one student)
This is the Student entity class:
#Entity
#Table(name="STUDENT")
public class Student {
#OneToMany
#JoinTable(name="student_hobbies", joinColumns=#JoinColumn(name="student_id"),
inverseJoinColumns=#JoinColumn(name="hobby_id"))
private Collection<Hobby> hobbies;
#Id
#Column(name="ID")
#GeneratedValue
private Integer id;
#Column(name="NAME")
private String name;
#Column(name="LASTNAME")
private String lastName;
// Getters and setters here
}
This is the Hobby entity class:
#Entity
#Table(name="HOBBY")
public class Hobby {
#OneToMany
#JoinTable(name="student_hobbies", joinColumns=#JoinColumn(name="hobby_id"),
inverseJoinColumns=#JoinColumn(name="student_id"))
private Collection<Student> students;
#Id
#Column(name="ID")
#GeneratedValue
private Integer id;
#Column(name="HOBBY_NAME")
private String hobby_name;
// Getters and setters here
}
Now I would like to implement the following DAO, but I don't know exactly how to do:
public interface MyDAO {
public void addHobbyForStudent(int student_id, int hobby_id);
public void RemoveHobbyForStudent(int student_id, int hobby_id);
}
Should I create an other entity class for the Join Table? Could someone give me some indication on the way to follow?
You can create an entity class for the join table, but you don't need to and you probably shouldn't. This will just create more code to maintain. You should create an entity for the join table if there is data in the join table you need to query. EG: if there was a start_time or something in the join table and you wanted to be able to see the start_time.
You should be using the #ManyToMany annotation when you're avoiding a join table entity. The documentation I linked to gives some good examples of how to do this:
Example 1:
// In Customer class:
#ManyToMany
#JoinTable(name="CUST_PHONES")
public Set<PhoneNumber> getPhones() { return phones; }
// In PhoneNumber class:
#ManyToMany(mappedBy="phones")
public Set<Customer> getCustomers() { return customers; }
Example 2:
// In Customer class:
#ManyToMany(targetEntity=com.example.PhoneNumber.class)
public Set getPhones() { return phones; }
// In PhoneNumber class:
#ManyToMany(targetEntity=com.example.Customer.class, mappedBy="phones")
public Set getCustomers() { return customers; }
Example 3:
// In Customer class:
#ManyToMany
#JoinTable(name="CUST_PHONE",
joinColumns=
#JoinColumn(name="CUST_ID", referencedColumnName="ID"),
inverseJoinColumns=
#JoinColumn(name="PHONE_ID", referencedColumnName="ID")
)
public Set<PhoneNumber> getPhones() { return phones; }
// In PhoneNumberClass:
#ManyToMany(mappedBy="phones")
public Set<Customer> getCustomers() { return customers; }
Related
Really confused by how one to many works in JPA, all the documents that I read, uses both one to many and many to one in their example, and I don't know if they are necessary or not, and it doesn't work when I tried it.
My question is, suppose I have two tables, and I want to populate the College object using findCollegeData() method, so that all the student in this college are in a list when I initialize the object.
Below is my approach, I am able to store all the students in the college list using storeCollegeData() method, but I am not able to retrieve the college object fully, the student list is always empty, even though the data is in the database, and it works if I try to search for student using college name directly.
public static EntityManager entityManager = something;
#Entity
#Data
#NoArgsConstructor
#AllArgsConstructor
public College {
#Id
#GeneratedValue(strategy= GenerationType.AUTO)
private int cId;
private String collegeName;
private int numOfStudent;
#OneToMany(mappedBy="collegeName", cascade=CascadeType.ALL, orphanRemoval=true)
private List<Student> studentList = new ArrayList<>();
}
#Entity
#Data
#NoArgsConstructor
#AllArgsConstructor
public Student {
#Id
#GeneratedValue(strategy= GenerationType.AUTO)
private int sId;
private String name;
private String collegeName;
private String city;
}
// college.getStudentList is always empty and I don't know why
public findCollegeData(String collegeName) {
College college = entityManager.find(College.class, collegeName);
}
// Student data in the studentList are inserted into student table
public storeCollegeData(College college) {
entityManager.persist(college);
}
// This method works
public findStudent(String collegeName) {
CriteriaBuilder cb = provider.get().getCriteriaBuilder();
CriteriaQuery<Student> query = cb.createQuery(Student.class);
Root<Student> student = query.from(Student.class);
query.where(
cb.and(
cb.equal(student.get("collegeName"), collegeName)
)
);
JobStatisticDB Student = provider.get().createQuery(query).getSingleResult();
}
Am i missing something??? Is join more appropriate than map here??? I dont know wat to do man
EDITED:
Got it to work by changing both of the collegeName as the primary key of table by adding #Id annotation, however though, how can I add an sId and cId to the table, so they can have duplicate college name???? Right now, I can't have duplicate college with the same name, and student that that goes to the same college!
Final Edited:
Changed database design to use foreign key see solution below
The accepted answer is incorrect: you define relationships between entities. The mappings should be as below for a bi-directional #OneToMany
College:
#Entity
#Data
#NoArgsConstructor
#AllArgsConstructor
public College {
#Id
#GeneratedValue(strategy= GenerationType.AUTO)
private int cId;
private String collegeName;
private int numOfStudent;
#OneToMany(mappedBy="college", cascade=CascadeType.ALL, orphanRemoval=true)
private List<Student> studentList = new ArrayList<>();
}
Student:
#Entity
#Data
#NoArgsConstructor
#AllArgsConstructor
public Student {
#Id
#GeneratedValue(strategy= GenerationType.AUTO)
private int sId;
private String name;
private String city;
//student table has a FK column college_id
#ManyToOne
#JoinColumn(name = "college_id")
private College college;
}
EntityManager find() takes the PK as an argument:
public findCollege(int collegeId) {
College college = entityManager.find(College.class, collegeId);
college.getStudents(); //will be populated
}
public findStudent(int studentId) {
Student student = entityManager.find(Student.class, studentId);
student.getCollege(); //will be populated
student.getCollege().getStudents(); //will be populated
}
If you want to find a college by name create a JPQL or Criteria query:
The field you reference in mappedBy must contain a value that equates to College's id field. Change it to collegeName instead of city, and it should work.
I have a (simplified) table structure that looks something like that:
customer table:
id name
-------------------
1 customer1
alias table:
customer_id alias
-------------------------------
1 customer one
1 customer uno
When I run the following query I easily get the list of aliases per customer:
select * from customer_alias where customer_id=1;
I would like to use this query in my hibernate to populate a list of type String. I tried using #Formula as follows:
#Entity
#Table(name = "customer")
public class Customer {
#Id
#Column(name = "id")
#GeneratedValue(strategy= GenerationType.AUTO)
private Long id;
#Column(name="name")
private String name;
#Formula("(select alias from customer_alias where customer_id = id)")
private List<String> aliases;
// Getters, setters, etc...
}
It didn't work and I got this exception:
Could not determine type for: java.util.List, at table: customer, for columns: [org.hibernate.mapping.Formula( (select alias from customer_alias where customer_id = id) )]
Is there anyway to achieve this? Doesn't have to be with #Formula of course. Any reasonable way would be great.
Here is an SQLFiddle of my example
You could use #ElementCollection for having a List of related aliases without the need to map the whole entity:
#ElementCollection
#CollectionTable(name = "customer_alias", joinColumns = #JoinColumn(name = "customer_id") )
#Column(name = "alias")
private List<String> aliases;
See also:
Difference between #OneToMany and #ElementCollection?
I think you don't want to use OneToMany annotation as the second table is just a list of strings you want to find something more elegant that would not require me to create another entity.
You can use #ElementCollection as below:
#Entity
#Table(name="college")
class College implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#Column(name="college_id")
#GeneratedValue(strategy=GenerationType.IDENTITY)
private int collegeId;
#Column(name="name")
private String collegeName;
#ElementCollection
#CollectionTable(name="student", joinColumns=#JoinColumn(name="college_id"))
#Column(name="student_name")
private Set<String> students;
public College() {
}
public Set<String> getStudents() {
return students;
}
public void setStudents(Set<String> students) {
this.students = students;
}
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;
}
#Override
public String toString() {
return "College [collegeId=" + collegeId + ", collegeName=" + collegeName + ", students=" + students + "]";
}
}
I don't think #Formula annotation supports collection it can only be applied for single-valued properties. Can't say if if there exists any tweak.
I have a function that merges two tables, each of these tables has a column that I want to filter.
#Entity
public class Contacts {
#Id
private int id;
#ManyToOne //reference user_id = id
private User user;
#ManyToOne //reference people_id = id
private People people;
//getters and setters
}
#Entity
public class User {
private int id;
private int name;
private Enterprise enterprise;
//getters and setters
}
#Entity
public class People {
private int id;
private int name;
//..others fields
private Enterprise enterprise;
//getters and setters
}
I need to list all "Contacts" where my enterprise id = 1. On a simple select (SQLServer), it will be:
SELECT c.* FROM CONTACTS c
INNER JOIN User u ON u.id = c.user_id
INNER JOIN People p on p.id = p.people_id
WHERE u.empresa_id = 1
I can't figure out how to do it with Criteria API, I already tried the follow code, but I keep receiving an error.
//code..public List<Obj> list(int id) {
Criteria crit = session.createCriteria(Contacts.class);
crit.add(Restrictions.eq(user.enterprise.id, id)); //it doesn't work!
crit.list();
}
org.hibernate.QueryException: could not resolve property: user.enterprise.id of: sys.com.model.Contacts
here i am writing code for sample using criteria.
public List<Student_document> getUserById(int id) {
Criteria criteria = sessionFactory.getCurrentSession().createCriteria(
Student_document.class);
criteria.add(Restrictions.eq("user_document.user_Id", id));
return criteria.list();
}
In hibernate, when I use the joined strategy. does hibernate support polymorphism?
for example:
#Entity
#Table(name = "PERSON")
#Inheritance(strategy=InheritanceType.JOINED)
public class Person {
#Id
#GeneratedValue
#Column(name = "PERSON_ID")
private Long personId;
#Column(name = "FIRSTNAME")
private String Fullname;
public Person() {
}
public Person(String fullname) {
this.Fullname= fullname
}
}
and the derived class:
#Entity
#Table(name="EMPLOYEE")
#PrimaryKeyJoinColumn(name="PERSON_ID")
public class Employee extends Person {
#Column(name="department_name")
private String departmentName;
public Employee() {
}
public Employee(String fullname, String departmentName,) {
super(fullname);
this.departmentName = departmentName;
}
}
also all the fields include the getter and setters.
so in my main, when I'll do this:
session.beginTransaction();
person e = new Employee();
e.setFullname("james");
e.setdepartmentName("R&D");
session.getTransaction().commit();
I know for a fact that if e was of Employee type, hibernate would have created a row for both Employee and Person tables.
but for this example will hibernate generate queries for person and employee?
in other words, will hibernate support the polymorphic behavior?
When I understand you question correctly than you want to know if hibernate does in that case automtically return Employees when you query for all Persons. Additionally if it inserts an Employee when the object is declared as Person p = new Employee().
Short answer to both yes. More detailed.
The insert operation is based on the actual type of the object and not on exactly the type written in your sourcecode.
Related to querying of Persons. Hibernate does left outer join all subtypes so you will get also Employess back when you do the following:
Query query = session.createQuery("From Person ");
List<Person> persons = query.getResultList();
I want to use one class to map three tables. I know javax.persistance provides the #SecondaryTable annotation to map two tables to one class.
Below is the code, where I have used #SecondaryTable. It allows me to define only one secondary table. But I need 3 tables to be used by the same class.
#Entity
#Table(name = "table1")
#SecondaryTable(name="table2")
public class TableConfig
implements Serializable {
/**
*
*/
private static final long serialVersionUID = 1L;
#Id
#Column(name = "mac", table= "table1")
private String uniqueIdentifier;
I want to use one class to map three tables, From what I know is that javax.persistance provides #SecondaryTable annotation to map two tables to one class
use #SecondaryTables to map more than one table.
You can map a single entity bean to several tables using the #SecondaryTables class level annotations. To express that a column is in a particular table, use the table parameter of #Column or #JoinColumn.
for example there is 3 entity's namely: Name , Address & Student:
Name entity will look like:
#Entity
#Table(name="name")
public class Name implements Serializable {
#Id
#Column(name="id")
private int id;
#Column(name="name")
private String name;
public Name(){}
public Name(int id,String name){
this.id=id;
this.name=name;
}
//getters and setters
}
Address entity will look like:
#Entity
#Table(name="address")
public class Address implements Serializable {
#Id
#Column(name="id")
private int id;
#Column(name="address")
private String address;
public Address(){}
public Address(int id, String address) {
super();
this.id = id;
this.address = address;
}
//getters and setters
}
Student entity will look like:
#Entity
#Table(name="student")
#SecondaryTables({
#SecondaryTable(name="name", pkJoinColumns={
#PrimaryKeyJoinColumn(name="id", referencedColumnName="student_id") }),
#SecondaryTable(name="address", pkJoinColumns={
#PrimaryKeyJoinColumn(name="id", referencedColumnName="student_id") })
})
public class Student implements Serializable {
#Id
#Column(name="student_id")
private int studentId;
#Column(table="name")
private String name;
#Column(table="address")
private String address;
public Student(){}
public Student(int studentId){
this.studentId=studentId;
}
//getters and setters
}
Store like:
Student s= new Student(1);
session.save(s);
Name n=new Name(s.getStudentId(),"Bilal Hasan");
session.save(n);
Address address = new Address(s.getStudentId(), "India");
session.save(address);
Student ob = (Student)session.get(Student.class, s.getStudentId());
System.out.println(ob.getStudentId());
System.out.println(ob.getName());
System.out.println(ob.getAddress());
ouput:
1
Bilal Hasan
India
you can define one class like below :
#Entity
#Table(name="table1")
#SecondaryTables({
#SecondaryTable(name="table2", pkColumnJoins={#PrimaryKeyJoinColumn(name = "id")}),
#SecondaryTable(name="table3", pkColumnJoins={#PrimaryKeyJoinColumn(name = "id")})
})
public class TestEntity {
#Id
#GeneratedValue
private int id;
private String field1;
#Column(name="column2", table="table2")
private String field2;
#Column(name="column3", table="table3")
private String field3;
getter and setter...
}
In your DB, should has three table, and all of them should has the same primary key "id".
then, use can test like this:
TestEntity test = new TestEntity();
test.setField1("field1");
test.setField2("field2");
test.setField3("field3");
em.merge(test);
after test, in your DB, you will find one record in each table:
table1:
1, field1
table2:
1, field2
table3:
1, field3
all of them will share the primary key value. Hope this will help you.
In Hibernate mapping file you can specify the entity-name mapping with virtual name along with polymorphism="explicit" and class name would be physical class name. Like that you may do multiple mappings. While loading the object use entityname (virtual name).