Merge works without cascade=CascadeType.Merge ?

[Himai Minh]

Hi, to my surprise, this merge works even though the cascade=CascadeType.Merge is not there.

@Entity
public class Department {
    @Id
    private int id;
    private String name;
    @OneToMany(mappedBy="department")
    private Collection<Employee> employees;
   ....

@Entity
public class Employee {
    @Id
    private int id;
    private String name;
    private long salary;
    
    @ManyToOne
    private Department department;
    
    @OneToOne  
    //(cascade=CascadeType.MERGE)
    //Why merging a detached entity of phone still works without this CascadeType.Merge?
     private Phone phone;
     .....

@Entity
public class Phone {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String phonenumber;
....

@Stateless
	public class EmployeeService{
	    @PersistenceContext(unitName="EmployeeService")
	    protected EntityManager em;

	   ....
	    public void merge(Employee employee2){
	    	
	    	Department dept = em.find(Department.class, 3);
	    	employee2.setDepartment(dept);
                 
	    	 Phone defaultphone = new Phone();
	    	 defaultphone.setPhonenumber("0000000000");
	    	  employee2.setPhone(defaultphone);
	    	  em.merge(employee2);
	    }

         ....

//In the servlet class, this is a code like this
 if (action.equals("Merge")){
	Employee employee2 = new Employee();
	employee2.setId(2);
	employee2.setName("Default");
	long zerosalary = 0;
	employee2.setSalary(zerosalary);
    empService.merge(employee2);	
}

Suppose I want to replace the following Gloria's record by the new employee2's data.
Originally, Gloria is from department 1 with phone number id =2 , like this:

//Assume there is a phone table with id=2 and phone number is "999-099-9999"
insert into employee (id,name, salary, department_id, phone_id) values (2,"Gloria", 2000, 1, 2);

After I execute the "Merge",
the record becomes 2, default, 0, 3, 5 for id, name, salary, department_id, phone_id.

A phone id=5 with values 000-000-0000  is created and then merged to Gloria's record.

To my surprise, why the phone number can be merged successfully without cascadeType.Merge in the phone attribute in Employee?

[Frits Walraven]

I haven't tried your example but I am also surprised. I had a look at the JPA specifications and I think it might be because of this last part:

3.2.7.1 Merging Detached Entity State
.... left out the beginning ....

If X is an entity merged to X', with a reference to another entity Y, where cascade=MERGE or cascade=ALL is not specified, then navigation of the same association from X' yields a reference to a managed object Y' with the same persistent identity as Y.

What do you think?

[Himai Minh]

I think Glassfish does not follow the spec.

If it follows the spec,
Employee  X has a child Phone Y. If it is merged to another Employee X' with Phone Y' in the persistence context, the Phone Y should not be merged with the Y' of X'.

[Frits Walraven]

Yes, EclipseLink (default JPA provider) doesn't follow the specifications on this point.

I have tested your example and I got to the same output: the Phone entity was persisted (!?!?).  Instead the persistence provider should have thrown an IllegalStateException, just like when you change line 15 of EmployeeService into em.persist(employee2);

If have reread the following part a couple of times and I am pretty sure they made a mistake implementing it:

If X is an entity merged to X', with a reference to another entity Y, where cascade=MERGE or cascade=ALL is not specified, then navigation of the same association from X' yields a reference to a managed object Y' with the same persistent identity as Y.

What is meant here (well the specs are not clear, but that is how I understand it) is that if there is no cascade specified that the only attribute that will be updated is the foreign key column to Y in X. Changes to Y won't be merged, but changes in X with regard to the relationship of Y will be updated in the database. Precondition here is that there is a row in the database reflecting Y.

Following your example:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		...
		String action = request.getParameter("action");
		if (action.equals("Merge")){
                        
			Department p = new Department();
			p.setId(3);
			p.setName("HeadDepartment");
			service.persistDepartment(p);
			
            // Phone gets ID=1 by the persistence provider
			Phone defaultphone = new Phone();
			defaultphone.setPhonenumber("0000000000");
			service.persistPhone(defaultphone);
			
		    Employee employee2 = new Employee();
		    employee2.setId(2);
		    employee2.setName("Default");
		    long zerosalary = 0;
		    employee2.setSalary(zerosalary);
		    
		    Employee notManaged = service.merge(employee2);
           
		    Phone phone = new Phone();
		    phone.setId(1);
		    phone.setPhonenumber("3333333333");
            // Note that there is a Phone with ID=1 in the database, so the detached employee (i.e. notManaged) should be updated when calling merge
		    notManaged.setPhone(defaultphone);
		    service.merge(notManaged);
		}
		...
}

and

@Stateless
public class EmployeeService {
	@PersistenceContext(unitName = "EmployeeService")
	protected EntityManager em;

	public Employee merge(Employee employee2) {
		Department dept = em.find(Department.class, 3);
		employee2.setDepartment(dept);
		return em.merge(employee2);
	}
	
	public void persistDepartment(Department p){
		em.persist(p);
	}
	
	public void persistPhone(Phone p){
		em.persist(p);
	}
}

When you run this you will see that the call to service.merge(notManaged); will update the foreign key column EMPLOYEE.PHONE_ID, but it won't change the PHONE.PHONENUMBER.

[Himai Minh]

Hi, Frits,
Thanks so much for your suggestion.
I tried your code and I tried both cases with and without line 22 emp.persist(phone) (to persist the phone number 33333333);

if (action.equals("Merge")){
	Department dep=new Department();
	dep.setId(100);
	dep.setName("Headquarter");
	empService.persistDepartment(dep);
	
	Phone defaultPhone= new Phone();
	defaultPhone.setPhonenumber("0000000000");
	empService.persistPhone(defaultPhone);
	
	Employee employee2 = new Employee();
	employee2.setId(2);
	employee2.setName("Default");
	long zerosalary = 0;
	employee2.setSalary(zerosalary);
	employee2.setPhone(defaultPhone);
	
    Employee notManaged = empService.merge(employee2);	
    Phone phone = new Phone();
    phone.setPhonenumber("3333333333");
     // I added this line
   // empService.persistPhone(phone);
 
   notManaged.setPhone(phone);
    empService.merge(notManaged);

public class Employee {
    @Id
    private int id;
    private String name;
    private long salary;
    
    @ManyToOne
    private Department department;
    
    @OneToOne (cascade=CascadeType.MERGE)
   
    private Phone phone;
   ....

Originally the data are (2, Gloria, 5000, 3, 2)
I got this change in Employee table:

id      name             salary        department_id      phone_id
-----------------------------------------------------------------------
2       Default               0                    100            12

when Phone table is showing:

id        phonenumber
--------------------------
2            3031231234
10          0000000000
12          3333333333

 
So, my result is the employee Default has phonenumber changed from  3031231234 to 000000000 and then to 3333333.

[Himai Minh]

Hi, Frit, I made another change to set the id of the phone:

if (action.equals("Merge")){
	Department dep=new Department();
	dep.setId(100);
	dep.setName("Headquarter");
	empService.persistDepartment(dep);
	
	Phone defaultPhone= new Phone();
	defaultPhone.setId(3);
	defaultPhone.setPhonenumber("0000000000");
	empService.persistPhone(defaultPhone);
	
	Employee employee2 = new Employee();
	employee2.setId(2);
	employee2.setName("Default");
	long zerosalary = 0;
	employee2.setSalary(zerosalary);
	employee2.setPhone(defaultPhone);
	
    Employee notManaged = empService.merge(employee2);	
    Phone phone = new Phone();
    phone.setId(3);
    phone.setPhonenumber("3333333333");
  
    notManaged.setPhone(phone);
    empService.merge(notManaged);
}

public class Employee {
    @Id
    private int id;
    private String name;
    private long salary;
    
    @ManyToOne
    private Department department;
    
    @OneToOne (cascade=CascadeType.MERGE)
    
    
    private Phone phone;

//I don't let the provider to auto increase the id. I need to call setID myself.
public class Phone {
@Id
 //@GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;

Output:

id  name  salary department_id phone_id
------------------------------------------------
2   Gloria  5000     1000                3

//Explanation: the phone number is merged as the CascadeType.Merge is specified for the phone. That is why it is changed from 0000000 to 3333333.
id phonenumber
-------------------
1    2021231234
2    3031234567
3    33333333333

[Himai Minh]

Hi, I made a small change in the employee  by removing the CascadeType.MERGE for the phone:

public class Employee {
    @Id
    private int id;
    private String name;
    private long salary;
    
    @ManyToOne
    private Department department;
    
    @OneToOne 
   //(cascade=CascadeType.MERGE)
   private Phone phone;

The merge method keeps the same:

 if (action.equals("Merge")){
	Department dep=new Department();
	dep.setId(100);
	dep.setName("Headquarter");
	empService.persistDepartment(dep);
	
	Phone defaultPhone= new Phone();
	defaultPhone.setId(3);
	defaultPhone.setPhonenumber("0000000000");
	empService.persistPhone(defaultPhone);
	
	Employee employee2 = new Employee();
	employee2.setId(2);
	employee2.setName("Default");
	long zerosalary = 0;
	employee2.setSalary(zerosalary);
	employee2.setPhone(defaultPhone);
	
    Employee notManaged = empService.merge(employee2);	
    Phone phone = new Phone();
    phone.setId(3);
    phone.setPhonenumber("3333333333");
 
    notManaged.setPhone(phone);
    empService.merge(notManaged);

// Explanation : The phone number is not changed from 00000000 to 3333333 as the phone (id=3) is not persisted and not merged to the original phone.
phone  table
id    phonenumber
---------------------
3      0000000000

id name salary department_id phone_id
-----------------------------------------------
2  default 50000  100                3

[Himai Minh]

Background:
Department and employee has a one to many relationship.
employee and phone has a one to one relationship.

Department is merged as it is a parent of employee.
But phone may or may not be merged as it is a child of employee.

Gloria 's department is 2 and phone number is 2021231234.

So far, we have 4 merge cases:

case 1:
         
Phone has cascade type= merge or not merge    
employee2 has phone number set to 0000000
employee2 merge happens. Department is changed to 100.
Gloria's phone number is set to 0000000

case 2:
Phone has cascade type = merge
employee2 has phone number id=10 set to 000000000.
employee2 merge happens.
notManaged employee is set to a new phone id=12 with number 333333333.
notManaged merge happens. Department is changed to 100.
Gloria's phone number is set to 333333333.

case 3:
Phone has cascade type = merge
employee2 has phone number id=3 set to 00000000.
notManaged is set to a new phone id=3  with number 33333333.
notManaged merge happens. Department is changed to 100.
Gloria's phone number is set to 33333333.

case 4:
phone has cascade type = not merge
employee2 has phone number id=3 set to  00000000.
notManaged is set to a new phone id=3 with number 33333333.
notManaged merge happens. Department is changed to 100.
Gloria's phone number is set to 0000000.

[Frits Walraven]

The merge bug was already reported some time ago: https://bugs.eclipse.org/bugs/show_bug.cgi?id=247662

Added our discussion as a comment to it and voted it to be solved.