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This message was imported from the former db4o newsgroup.

Hi.

I am making a testing program with two classes: Course and Student.  I am 
using a server-client connection. I have cascadeOnUpdate(true) in the two 
classes.

I am using a vector in each class to make the relationship. In the class 
Student the method addCourse() allow to register a student to a course and 
this method  use Course.addStudent() to keep synchronize de relationship.

Every thing is working nice until here.

I was trying to use objectCanDelete() on the class student to delete the 
object student from the vector students in the class Course to keep 
synchronize the relationship. But, for some reason I could not find the way 
to make it to work.

I will like you help to understand how to use objectCanDelete() call in 
server in this little test program.

Thank you.

----------------------------
Classes

----------------------------
package dataoo7;
import java.util.*;
import com.db4o.*;

public class Course {
 private String name;
 private Vector students;

 public Course (String name) {
  this.name=name;
  this.students = new Vector();
 }
 public void setName(String name) {
  this.name = name;
 }
 public String getName() {
  return this.name;
 }
 public void addStudent(Student newstudent) {
  this.students.add( (Student)newstudent );
 }
 public Vector getStudents(){
  return this.students;
 }
 public int numStudents(){
  return this.students.size();
 }

 public String toString() {
  return this.name+"("+this.students.size()+")";
 }
}

----------------------------------
package dataoo7;
import java.util.*;
import com.db4o.*;

public class Student {
 private String name;
 private Vector courses;

 public Student (String name) {
  this.name=name;
  this.courses = new Vector();
 }
 public void setName(String name) {
  this.name = name;
 }
 public String getName() {
  return this.name;
 }
 public void addCourse(Course newcourse) {
  newcourse.addStudent( this );
  this.courses.add( (Course)newcourse );
 }
 public Vector getCourses(){
  return this.courses;
 }
 public int numCourses(){
  return this.courses.size();
 }

 public String toString() {
  return this.name+"("+this.courses.size()+")";
 }
}

This message was imported from the former db4o newsgroup.

Does any body have a little help for this question?



In other way: Can some body indicate what is the "question profile" 
necessary to follow to receive answers?



Thank you.




"Italo Osorio"  wrote in message 
news:d99a68$b7s$1@p15090641.pureserver.de...
> Hi.
>
> I am making a testing program with two classes: Course and Student.  I am 
> using a server-client connection. I have cascadeOnUpdate(true) in the two 
> classes.
>
> I am using a vector in each class to make the relationship. In the class 
> Student the method addCourse() allow to register a student to a course and 
> this method  use Course.addStudent() to keep synchronize de relationship.
>
> Every thing is working nice until here.
>
> I was trying to use objectCanDelete() on the class student to delete the 
> object student from the vector students in the class Course to keep 
> synchronize the relationship. But, for some reason I could not find the 
> way to make it to work.
>
> I will like you help to understand how to use objectCanDelete() call in 
> server in this little test program.
>
> Thank you.
>
> ----------------------------
> Classes
>
> ----------------------------
> package dataoo7;
> import java.util.*;
> import com.db4o.*;
>
> public class Course {
> private String name;
> private Vector students;
>
> public Course (String name) {
>  this.name=name;
>  this.students = new Vector();
> }
> public void setName(String name) {
>  this.name = name;
> }
> public String getName() {
>  return this.name;
> }
> public void addStudent(Student newstudent) {
>  this.students.add( (Student)newstudent );
> }
> public Vector getStudents(){
>  return this.students;
> }
> public int numStudents(){
>  return this.students.size();
> }
>
> public String toString() {
>  return this.name+"("+this.students.size()+")";
> }
> }
>
> ----------------------------------
> package dataoo7;
> import java.util.*;
> import com.db4o.*;
>
> public class Student {
> private String name;
> private Vector courses;
>
> public Student (String name) {
>  this.name=name;
>  this.courses = new Vector();
> }
> public void setName(String name) {
>  this.name = name;
> }
> public String getName() {
>  return this.name;
> }
> public void addCourse(Course newcourse) {
>  newcourse.addStudent( this );
>  this.courses.add( (Course)newcourse );
> }
> public Vector getCourses(){
>  return this.courses;
> }
> public int numCourses(){
>  return this.courses.size();
> }
>
> public String toString() {
>  return this.name+"("+this.courses.size()+")";
> }
> }
>

This message was imported from the former db4o newsgroup.

Italo Osorio :
> Does any body have a little help for this question?
> 
> In other way: Can some body indicate what is the "question profile" 
> necessary to follow to receive answers?

Well, you've said you've tried to use the objectCanDelete() callback, 
but in your example there is no objectCanDelete() to be found. There is 
no a method to delete the student at all in your code. So, provide the 
code which is not working, rather than the code which is working, then 
somebody might guess what is exactly wrong with your code. Do something 
like a "unit test", which should work, but apparently don't.

This message was imported from the former db4o newsgroup.

Hi Artem, thank you for you answer.

I didn't include my code because I was expecting that some body can propose 
one to see a "clear" approach to the problem with out my "inexperience code". 
Sorry for that.

Well my request is that:

> I will like you help to understand how to use objectCanDelete() call in 
> server in this little test program.

I was trying to do something like this:

Add to the Student Class this method.

public void delCourse(Course course){
 this.courses.remove(course);
}


Add to the Course Class this method.

public void objectCanDelete(ObjectContainer container    ) {
 Student tmpStudent;
 for( int i=0; i < this.students.size(); i++ ) {
  tmpStudent = (Student) this.students.get(i);
  tmpStudent.delCourse(this);
  container.set(tmpStudent);
}
return true;
}


This code works with our errors but it does not delete a course from the 
vector in student class when I deleted a course.

Thank you for you help.

Italo.


"Artem Gr"  wrote in message 
news:d9rg7a$k1u$1@p15090641.pureserver.de...
> Italo Osorio :
>> Does any body have a little help for this question?
>>
>> In other way: Can some body indicate what is the "question profile" 
>> necessary to follow to receive answers?
>
> Well, you've said you've tried to use the objectCanDelete() callback, but 
> in your example there is no objectCanDelete() to be found. There is no a 
> method to delete the student at all in your code. So, provide the code 
> which is not working, rather than the code which is working, then somebody 
> might guess what is exactly wrong with your code. Do something like a 
> "unit test", which should work, but apparently don't.

This message was imported from the former db4o newsgroup.

I suspect the following problem: since in client-server environment 
'objectCanDelete' callback is executed on the server, the 'course' is 
probably removed on the server, but remains in the 'student' instances 
on the client.

You can check if this is the case by using ExtObjectContainer#refresh 
method on the student.

The simplest solution to this problem, i suppose, is to move the 
'course' deletion into the 'delCourse' method. Consider, also, using 
Db4oList instead of Vector.
http://db4objects.com/community/ontheroad/apidocumentation/com/db4o/types/Db4oList.html

> 
> I was trying to do something like this:
> 
> Add to the Student Class this method.
> 
> public void delCourse(Course course){
>  this.courses.remove(course);
> }
> 
> 
> Add to the Course Class this method.
> 
> public void objectCanDelete(ObjectContainer container    ) {
>  Student tmpStudent;
>  for( int i=0; i < this.students.size(); i++ ) {
>   tmpStudent = (Student) this.students.get(i);
>   tmpStudent.delCourse(this);
>   container.set(tmpStudent);
> }
> return true;
> }
> 
> 
> This code works with our errors but it does not delete a course from the 
> vector in student class when I deleted a course.
>

This message was imported from the former db4o newsgroup.

Hi Artem. Thank you for you help.

> I suspect the following problem: since in client-server environment 
> 'objectCanDelete' callback is executed on the server, the 'course' is 
> probably removed on the server, but remains in the 'student' instances on 
> the client.

The documentation about objectCanDelete said: In a client/server setup this 
callback method will be executed on the server.
The question is how to access other objects when is "executed on the server" 
by the Call back methods.

In Student Class I add this method.

 public void delCourse(int pos, ObjectContainer container){
  Course course = (Course) this.courses.get(pos);
  course.delStudent(this);
  container.set(course);
  this.courses.removeElementAt( pos );
 }

I use this method to delete a Student Course by passing POS with the vector 
position of the course which I want to delete and the object container. The 
goal is to delete a student object from the vector "students" in the Course 
class. The idea it is not to delete the Course, only to "unregister" a 
student from this course. This method works fine and maybe is because it is 
"executed on the client".


The other point is that if I whan to deleted a Course, before that, I need 
to inform the students objects they are not any more register in the Course. 
I know what student are register in the class by the Course.students Vector. 
I need to go to each of this students and Delete from thir Course vector the 
Course with I wanto to delete. It need to be done by objectCanDelete. I can 
do that in the JSP but it is not the optimal, need to be done in the object.


> You can check if this is the case by using ExtObjectContainer#refresh 
> method on the student.

I am using JSP. I have one jsp which show the courses and students. Another 
JSP delete the objects.

This are my classes:


//----------------------------------------------
package dataoo7;

import java.util.*;
import com.db4o.*;


public class Course {
 private String id;
 private String name;
 private Vector students;


 public Course (String id,String name) {
  this.id=id;
  this.name=name;
  this.students = new Vector();
 }

 public String getId() {
  return this.id;
 }

 public void setName(String name) {
  this.name = name;
 }

 public String getName() {
  return this.name;
 }

 public void addStudent(Student newstudent) {
  this.students.add( (Student)newstudent );
 }

 public Vector getStudents(){
  return this.students;
 }

 public int numStudents(){
  return this.students.size();
 }


 public String toString() {
  return this.name+"("+this.students.size()+")";
 }

 public void delStudent(Student student){
  this.students.remove( student );
 }

}

//---------------------------------------------------
package dataoo7;

import java.util.*;
import com.db4o.*;


public class Student {
 private String id;
 private String name;
 private Vector courses;


 public Student (String id, String name) {
  this.id=id;
  this.name=name;
  this.courses = new Vector();
 }

 public String getId() {
  return this.id;
 }

 public void setName(String name) {
  this.name = name;
 }

 public String getName() {
  return this.name;
 }

 public void addCourse(Course newcourse) {
  newcourse.addStudent( this );
  this.courses.add( (Course)newcourse );
 }

 public Vector getCourses(){
  return this.courses;
 }

 public int numCourses(){
  return this.courses.size();
 }


 public String toString() {
  return this.name+"("+this.courses.size()+")";
 }

 public void delCourse(int pos, ObjectContainer container){
  Course course = (Course) this.courses.get(pos);
  course.delStudent(this);
  container.set(course);
  this.courses.removeElementAt( pos );
 }

 public boolean objectCanDelete(ObjectContainer container) {
  Course course;
  for( int i=0; i < this.courses.size(); i++ ) {
   course = (Course) this.courses.get(i);
   course.delStudent(this);
   container.set(course);
  }
  return true;
 }

}

This message was imported from the former db4o newsgroup.

> The other point is that if I whan to deleted a Course, before that, I need 
> to inform the students objects they are not any more register in the Course. 
> I know what student are register in the class by the Course.students Vector. 
> I need to go to each of this students and Delete from thir Course vector the 
> Course with I wanto to delete. It need to be done by objectCanDelete. I can 
> do that in the JSP but it is not the optimal, need to be done in the object.

If i am not mistaken, you can't, currently, access client objects from 
the server, only the other way around: you can access server objects 
from the client. Therefore, you /can/ change something from 
objectCanDelete, but then you should use ExtObjectContainer#refresh or 
ExtObjectContainer#peekPersisted or ExtObjectContainer#purge(Object) to 
refresh client data from the server.

This message was imported from the former db4o newsgroup.

Hi.. I am really confuse.

What is the use of "objectCanDelete" if I can not access the information 
form the object which I am deleting? What is the use of "objectCanDelete" if 
I can't access data from other classes to keep data integrity?

Sorry if I am missing something in my understanding. I have no problem with 
the other Callbacks.

Could you give me a code example of how to use a "objectCanDelete"?

Thaks you.



"Artem Gr"  wrote in message 
news:d9tplr$1mi$1@p15090641.pureserver.de...
>
>> The other point is that if I whan to deleted a Course, before that, I 
>> need to inform the students objects they are not any more register in the 
>> Course. I know what student are register in the class by the 
>> Course.students Vector. I need to go to each of this students and Delete 
>> from thir Course vector the Course with I wanto to delete. It need to be 
>> done by objectCanDelete. I can do that in the JSP but it is not the 
>> optimal, need to be done in the object.
>
> If i am not mistaken, you can't, currently, access client objects from the 
> server, only the other way around: you can access server objects from the 
> client. Therefore, you /can/ change something from objectCanDelete, but 
> then you should use ExtObjectContainer#refresh or 
> ExtObjectContainer#peekPersisted or ExtObjectContainer#purge(Object) to 
> refresh client data from the server.

This message was imported from the former db4o newsgroup.

Obviously, objectCanDelete callback should be used to prevent some 
objects from being deleted.

> Hi.. I am really confuse.
> 
> What is the use of "objectCanDelete" if I can not access the information 
> form the object which I am deleting? What is the use of "objectCanDelete" if 
> I can't access data from other classes to keep data integrity?
> 
> Sorry if I am missing something in my understanding. I have no problem with 
> the other Callbacks.
> 
> Could you give me a code example of how to use a "objectCanDelete"?

hi <br />
I just don't get it. the use for objectCanDelete is quite simple to understand but, the use of objectOnDelere I don't get:
<p />
I would like to delete some children of a Object which is about to be deleted.
<p />
The problem here is that when I enter the objectOnDelete function all child objects are NULL, and I can't make my custom deletion logic
<p />
Ie. <br />
The Object which is about to be deleted has a List of child objects. When the object is deleted I want to run through the list and delete each element, or more complex determin if the element has to be deleted or the reference just should be deleted..



<p />
Can anybody explain how to handle more complex treestructure deletion...

<p />
TIA<br />
/Kasper

f_kasper wrote:

I just don't get it. the use for objectCanDelete is quite simple to understand but, the use of objectOnDelere I don't get: I would like to delete some children of a Object which is about to be deleted. The problem here is that when I enter the objectOnDelete function all child objects are NULL, and I can't make my custom deletion logic

Did you try activating the object in the objectOnDelete() callback method?

 

Hi
Thanks for the suggestion. I now have tried to activate the object in following code, without any change. All child objects (objectList) are still NULL.

(All child members are present when retriving the object from db4o but in the following 2 functions all child objects are null)

I just don't get it. All examples in the forum and the documentation suggest that this is just the way to go. I am fairly new to db4o so there might be some obvious setting i am missing?


public class MyObject{
private List objectList; //this is my childObject
private String name;

public boolean objectCanDelete(ObjectContainer container){
container.activate(this,2);
System.out.println("canDelete object: "+objectList);
for(Object object:objectList)
container.delete(object);
return true;
}

public void objectOnDelete(ObjectContainer container){
// called after an object was deleted.
container.activate(this,2);
System.out.println("onDelete: "+objectList);
for(Object object:objectList)
container.delete(object);
}
...
}


When run the program returns:
canDelete object: null
onDelete: null

I'm not sure, but there might be a safety check in there which would prevent activate(this) from working in a callback. Carl or Patrick would know for sure. From what I see, that looks correct. It's possible that the bug is elsewhere in your code. Maybe the list isn't getting stored in the first place?

---

Eric Falsken » db4objects

The Problem still exists, here is a simple TestCase:

import com.db4o.*;
import com.db4o.ext.*;

import java.io.*;

public class Test {
    long i;
    String s;


    public static void main(String[] args) throws IOException, InterruptedException {
        new File("test.yap").delete();
        ObjectServer server = Db4o.openServer("test.yap", 0);
        ExtClient c = (ExtClient) server.openClient();
        Test t = new Test();
        t.i= 1;
        t.s= "foo";
        c.set(t);
        c.commit();

        c.delete(t);
        c.commit();

        while (!c.close());
        while (!server.close());
    }

    public void objectOnDelete(ObjectContainer container) throws IOException {
        System.out.println(this.i + ", "+ this.s);
        // bug:
        //  expected    1, foo
        //  result:     0, null
    }
}

even with activation in the objectOnDelete method the Object values are not set up correctly.

cheers

Andreas 

I supplied a patch which fixes this bug:

http://tracker.db4o.com/browse/COR-741