In the following example, there are 3 entities which have relations e.g. #ManyToMany, #OneToMany and #ManyToOne:
Student:
#Entity
#Data
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
Long id;
private String name;
#JsonIgnore
#ManyToMany(mappedBy = "students")
private Set<Subject> subjects = new HashSet<>();
}
Subject:
#Entity
#Data
public class Subject {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
Long id;
private String name;
#ManyToMany
#JoinTable(
name = "subject_student",
joinColumns = #JoinColumn(name = "subject_id"),
inverseJoinColumns = #JoinColumn(name = "student_id")
)
Set<Student> students = new HashSet<>();
#ManyToOne(cascade = CascadeType.ALL)
#JoinColumn(name = "teacher_id", referencedColumnName = "id")
private Teacher teacher;
}
Teacher:
#Entity
#Data
public class Teacher {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
#JsonIgnore
#OneToMany(mappedBy = "teacher")
private Set<Subject> subjects;
}
1. In the subject entity, I tried to remove #JoinColumn and the related entities are connected as the example above:
#ManyToMany
#JoinTable(name="subject_student")
public Set<Student> students = new HashSet<>();
#ManyToOne(cascade = CascadeType.ALL)
private Teacher teacher;
So, if we want to use subject_id - student_id pair in subject_student table and use teacher_id in subject table as it is created in the example, can I use my simplified notation by removing #JoinColumn? Because, if there is not a special case, I think it is redundant to verbose notation of relations.
2. When I use the second approach, the columns are created as plural e.g. subjects_id - students_id in subject_student. So, can I prevent this and create them as in the previous example by using my approach?
Related
I have two Entities in my Spring-Boot mongoDb Application:
Course.java
#Entity
#Table(name = "students")
public class Course {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
Long id;
String title;
#Enumerated(EnumType.STRING)
OptionCourse optionCourse;
private double fee;
#ManyToMany(fetch = FetchType.LAZY,
cascade = {
CascadeType.PERSIST,
CascadeType.MERGE
},
mappedBy = "courses")
private Set<Student> student = new HashSet<>();
}
and
Student.java
Entity
#Table(name = "students")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
Long id;
int age;
String department;
#ManyToMany(fetch = FetchType.LAZY,
cascade = {
CascadeType.PERSIST,
CascadeType.MERGE
})
#JoinTable(name = "student_courses",
joinColumns = { #JoinColumn(name = "student_id") },
inverseJoinColumns = { #JoinColumn(name = "course_id") })
private Set<Course> courses = new HashSet<>();
}
for my mongoDb database:
I want to realise a Many-to-Many-Relationship between both Entities. Every courses should be able to assign multiple student.
How can i replace my annotations (#Entity, #Table,#Id, #Enumerated(EnumType.STRING) and #ManyToMany(fetch = FetchType.LAZY,...)) in my entities. I use spring-boot-starter-data-mongodb in my maven dependencies
I have two tables, student and teacher, with a relationship ManyToOne. The table structure is as follows
student(
id long,
student_id string,
....
teacher_id string,
active boolean
)
teacher(
id long,
teacher_id string,
....
active boolean
)
I'm using Spring boot and Hibernate. Here when updating an entity, the active column of the existing row in the table will be set to false and a new row will be added with a new id(long) and active as true. That is why there are two id values in each table. The problem here is I have specified the student-teacher relation as many to one in my entity with the foreign key as teacher_id.
#Entity
#Table(name = "student")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#Column(name = "student_id")
private String studentId;
#ManyToOne
#JoinColumn(name = "teacher_id", referencedColumnName = "teacher_id")
private Teacher teacher;
#Column(name = "active")
#JsonIgnore
private Boolean active = true;
}
#Entity
#Table(name = "teacher")
public class Teacher {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#Column(name = "teacher_id")
private String teacherId;
#OneToMany(mappedBy = "teacher")
private Set<Student> students = new HashSet<>();
#Column(name = "active")
#JsonIgnore
private Boolean active = true;
}
But since multiple teachers can occur with the same teacher_id, this fails. Is there any way to give a condition to the relationship to fetch the teacher with active as true? In table, there will be only one teacher with the given id and active as true.
I just came across your post. I had the same requirement few months ago and this is what i did..
public class Student {
#Column(name = "teacher_id ")
private String teacherId;
#ManyToOne
#JoinFormula(value = "(Select t.id from teacher t where t.teacher_id= teacher_id and t.active=1)"
)
private Teacher teacher;
}
#Entity
#Table(name = "teacher")
public class Teacher {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#Column(name = "active")
#JsonIgnore
private Boolean active = true;
}
This approach works for my case. If there is a better one you can share it.
Thanks
There should be no teacher_id foreign key field in the Teacher entity. Rather, just use the primary key id column instead. Consider this version of your entities:
#Entity
#Table(name = "student")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#Column(name = "student_id")
private String studentId;
#ManyToOne
#JoinColumn(name = "teacher_id", referencedColumnName = "id")
private Teacher teacher;
#Column(name = "active")
#JsonIgnore
private Boolean active = true;
}
#Entity
#Table(name = "teacher")
public class Teacher {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#OneToMany(mappedBy = "teacher")
private Set<Student> students = new HashSet<>();
#Column(name = "active")
#JsonIgnore
private Boolean active = true;
}
This design should enforce that a given student can be associated with only one teacher (though a given teacher can have multiple students).
I have two classes Student and Subject:
Student
#Entity
#Table(name = "students")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private int id;
#Column(name = "full_name")
private String fullName;
#ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy = "students")
private Set<Subject> subject;
//Getters & setters
}
Subject
#Entity
#Table(name = "subjects")
public class Subject {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private int id;
#Column(name = "subject_name")
private String name;
#ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
private Set<Student> students;
//Getters & setters
}
When I am trying to save a student registered with more than one subject, I am not getting any records in the JoinTable.
My code for saving the entiers:
Subject subject = new Subject();
subject.setName("JAVA");
Subject subject2 = new Subject();
subject2.setName("C");
Subject subject3 = new Subject();
subject3.setName("C++");
Student student = new Student();
student.setFullName("dibya");
Set<Subject> subjects = new HashSet<>();
subjects.add(subject2);
subjects.add(subject);
subjects.add(subject3);
student.setSubject(subjects);
session.saveOrUpdate(student);
Please tell me where am I doing wrong.
Found the solution to my problem.
It was not with the configuration. With just mappedBy attribute ManyToMany relationship can be accomplished.
In my program I have added session.saveOrUpdate(subject); after saving saving the students.
This solved my problem. :)
A many-to-many relationship is modeled as a join table. This is a table with 2 columns, both are foreign keys to the tables of the repationship (Student and Subject in your case).
In Hibernate if you use #ManyToMany you need to either #JoinTable or #JoinColumn to define the relationship. Once it's done your update will work.
See
http://docs.jboss.org/hibernate/orm/4.3/manual/en-US/html/ch07.html
and
http://www.dzone.com/tutorials/java/hibernate/hibernate-example/hibernate-mapping-many-to-many-using-annotations-1.html
EDIT:
Applied to your code it should look like (UNTESTED):
In Student:
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "student_id")
private int id;
....
#ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
#JoinTable(name = "students_subject", joinColumns = { #JoinColumn(name = "student_id") }, inverseJoinColumns = { #JoinColumn(name = "subject_id") })
private Set<Subject> subject;
In Subject
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "subject_id")
private int id;
...
#ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
#JoinTable(name = "students_subject", joinColumns = { #JoinColumn(name = "subject_id") }, inverseJoinColumns = { #JoinColumn(name = "student_id") })
private Set<Student> students;
Your case might be falling into mixing JPA and Hibernate cascading. Try annotate your #ManyToMany with additional
#Cascade({CascadeType.SAVE_UPDATE})
you can also consult this two links to check if this is really your case:
Confusion between JPA and Hibernate cascading
http://www.mkyong.com/hibernate/cascade-jpa-hibernate-annotation-common-mistake/
I have an existing database modeled the following way:
Student - SchoolId(PK), StudentId(PK), StudentName
Teacher - SchoolId(PK), TeacherId(PK), TeacherName
Student_Teacher - SchoolId(PK), StudentId(PK), TeacherId(PK)
Foreign key references exist from Student_Teacher to respective entities.
Now I am creating hibernate entities for this existing database. And I am running into weird issues creating Many-to-Many mapping from Student to Teacher.
#Entity
#Table(name = "Student")
public class Student {
#EmbeddableId
private StudentPK itemId;
#Column(name="StudentName")
private String studentName;
#ManyToMany(cascade = CascadeType.ALL)
#JoinTable(name="Student_Teacher", joinColumns={#JoinColumn(name="SchoolId", referencedColumnName="SchoolId"),#JoinColumn(name="StudentId", referencedColumnName="StudentId")}, inverseJoinColumns={#JoinColumn(name="SchoolId", referencedColumnName="SchoolId"),#JoinColumn(name="TeacherId", referencedColumnName="TeacherId")})
private List<Teacher> attachments=new ArrayList<Teacher>();
}
The above code compains about some duplicate SchoolId reference.
Any ideas?
As I see that there is an issue in your mapping of entities, It should be as follows
school - school_id(PK), school_name
student - student_id(PK) , student_name, fk_school_id(FK),
teacher - teacher_id(PK), teacher_name , fk_school_id(FK)
*student_teacher* - student_teacher_id(PK), fk_student_id(FK), fk_teacher_id(FK)
and Entity clasess as follows
School Entity
#Entity
#Table(name = "school")
public class School {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column (name = "school_id")
private int Id;
#Column(name="school_name")
private String schoolName;
#OneToMany(fetch = FetchType.LAZY, mappedBy = "school")
private Set<Student> students = new HashSet<Student>
#OneToMany(fetch = FetchType.LAZY, mappedBy = "school")
private Set<Teacher> teachers = new HashSet<Teacher>
}
Student Entity
#Entity
#Table(name = "student")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column (name = "student_id")
private int Id;
#Column(name="student_name")
private String studentName;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "school_id", nullable = false)
private School school;
#ManyToMany(cascade = CascadeType.ALL)
#JoinTable(name = "student_teacher", joinColumns = {#JoinColumn(name = "fk_student_id") }, inverseJoinColumns = { #JoinColumn(name = "fk_teacher_id") })
private List<Teacher> teachers = new ArrayList<Teacher>();
}
Teacher Entity
#Entity
#Table(name = "teacher")
public class Teacher {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column (name = "teacher_id")
private int Id;
#Column(name="teacher_name")
private String name;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "school_id", nullable = false)
private School school;
#ManyToMany(cascade = CascadeType.ALL)
#JoinTable(name = "student_teacher", joinColumns = {#JoinColumn(name = "fk_teacher_id") }, inverseJoinColumns = { #JoinColumn(name = "fk_student_id") })
private List<Student> students =new ArrayList<Student>();
}
hope this will solve this problem..
as you have declare 'SchoolId' as PK in Student_Teacher table it will not allow you to add duplicate entry for SchoolId field for Student_Teacher table and this is not the case. thus the above relationship will gives duplicate SchoolId reference. when you are going to add two different students from same school into Student_Teacher table..
Did you define the various PKs per entity as compound keys, as i note that you use multiple PKs per entity. Is there any constraint why you can't use a sole PK per entity and just use a relation table to bind the 2 entities?
I have the following entities
Student
#Entity
public class Student implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
//getter and setter for id
}
Teacher
#Entity
public class Teacher implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
//getter and setter for id
}
Task
#Entity
public class Task implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#ManyToOne(optional = false)
#JoinTable(name = "student_task", inverseJoinColumns = { #JoinColumn(name = "student_id") })
private Student author;
#ManyToOne(optional = false)
#JoinTable(name = "student_task", inverseJoinColumns = { #JoinColumn(name = "teacher_id") })
private Teacher curator;
//getters and setters
}
Consider that author and curator are already stored in DB and both are in the attached state. I'm trying to persist my Task:
Task task = new Task();
task.setAuthor(author);
task.setCurator(curator);
entityManager.persist(task);
Hibernate executes the following SQL:
insert
into
student_task
(teacher_id, id)
values
(?, ?)
which, of course, leads to null value in column "student_id" violates not-null constraint
Can anyone explain this issue and possible ways to resolve it?
UPDATE
See my own solution below.
I've resolved my issue with the help of #SecondaryTable and switched from #JoinTable to #JoinColumn:
Task
#Entity
#SecondaryTable(name="student_task")
public class Task implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#ManyToOne(optional = false)
#JoinColumn(table = "student_task", name = "student_id")
private Student author;
#ManyToOne(optional = false)
#JoinColumn(table = "student_task", name = "teacher_id")
private Teacher curator;
//getters and setters
}
Now, generated SQL looks like:
insert
into
student_task
(student_id, teacher_id, id)
values
(?, ?, ?)
and everything works just fine :)
I think you are missing the JoinColumns tag...
joinColumns = { #JoinColumn(name = "student_id", referencedColumnName = "id") }
joinColumns = { #JoinColumn(name = "teacher_id", referencedColumnName = "id") }
in author and curator respectively
Also remember, that the inversjoincolumn is the column in the owned table.. so it must be something like:
inverseJoinColumns = {#JoinColumn(name="id")})