#GeneratedValue annotation is not working on mapping table - spring

Question

I have a many to many relationship like below.
#Entity
#Table(name = "employees")
public class Employee {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "EMP_ID")
private int id;
private String firstName;
private String lastName;
#JoinTable(name = "employee_project_mapping", joinColumns = #JoinColumn(name = "EMPLOYEE_ID", referencedColumnName = "EMP_ID"), inverseJoinColumns = #JoinColumn(name = "PROJECT_ID", referencedColumnName = "PJT_ID"))
#JsonManagedReference
#ManyToMany(cascade = CascadeType.ALL)
Set<Project> projects = new HashSet<Project>();
.....
.....
}
#Entity
#Table(name = "projects")
public class Project {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "PJT_ID")
private int projectId;
private String projectName;
private int teamSize;
#ManyToMany(mappedBy = "projects")
#JsonBackReference
private Set<Employee> emps = new HashSet<Employee>();
.....
.....
}
#Entity
#Table(name = "employee_project_mapping")
public class EmployeeProjectMapping {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "EMP_PJT_ID")
private Integer empPjtId;
#Column(name = "PROJECT_ID")
private Integer projectId;
#Column(name = "EMPLOYEE_ID")
private Integer emploeeId;
.....
.....
}
But when I am trying to insert an employee object with set of projects, it is failing to create auto generated id for the column EMP_PJT_ID (this is an id to the mapping table record). Can't I add an auto generated id for mapping table using jpa?
Error trace
Hibernate: insert into employees (emp_id, first_name, last_name) values (null, ?, ?)
Hibernate: insert into employee_project_mapping (employee_id, project_id) values (?, ?)
2021-04-22 23:34:25.973 ERROR 24126 --- [nio-8080-exec-9] o.h.engine.jdbc.spi.SqlExceptionHelper : NULL not allowed for column "EMP_PJT_ID"; SQL statement:
insert into employee_project_mapping (employee_id, project_id) values (?, ?) [23502-200]
2021-04-22 23:34:25.975 ERROR 24126 --- [nio-8080-exec-9] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement] with root cause
org.h2.jdbc.JdbcSQLIntegrityConstraintViolationException: NULL not allowed for column "EMP_PJT_ID"; SQL statement:
insert into employee_project_mapping (employee_id, project_id) values (?, ?) [23502-200]

Answer 1

The mapping of a many-to-many should be:
#Entity
#Table(name = "employees")
public class Employee {
...
#OneToMany(mappedBy = "employee", cascade = CascadeType.ALL, orphanRemoval = true)
Set<EmployeeProjectMapping> projects = new HashSet<EmployeeProjectMapping>();
...
// Utility method to update both sides of the association
public void addProject(Project project) {
EmployeeProjectMapping empProject = new PersEmployeeProjectMappingonAddress( this, project );
projects.add( empProject );
project.getEmployees().add( empProject );
}
}
#Entity
#Table(name = "projects")
public class Project {
...
#OneToMany(mappedBy = "project", cascade = CascadeType.ALL, orphanRemoval = true)
private Set<EmployeeProjectMapping> emps = new HashSet<Employee>();
...
}
#Entity
#Table(name = "employee_project_mapping")
public class EmployeeProjectMapping {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "EMP_PJT_ID")
private Integer empPjtId;
#Id
#ManyToOne
#JoinColumn(name = "PROJECT_ID")
private Project project;
#Id
#ManyToOne
#JoinColumn(name = "EMPLOYEE_ID")
private Employee employee;
.....
.....
}
Make sure to check the example on the Hibernate ORM documentation for all the details.