Circular Object Reference (cache)

jwira · **Joined:** Mon Jul 21, 2008 2:52 am **Posts:** 8

I have a user table as follows

Code:

create table usr
(
UsrId int,
UsrCode varchar(8),
UpdatedBy int
)

with data

UsrId       UsrCode    UpdatedBy
----------- ---------- ----------- 
1           tusr1      1
2           tusr2      1


C# code
public class Usr
{
   protected int _usrId;
   protected string _usrCode;
   protected Usr _updatedBy;

   public virtual int Id
   {
      get {return _id;}
      set {_id = value;}
   }

   public virtual string UsrCode
   {
      get { return _usrCode; }
      set { _usrCode = value; }
   }

   public virtual Usr UpdatedBy
   {
      get { return _updatedBy; }
      set { _updatedBy = value; }
   }

}

Nhibernate.hbm

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
   <class name="Test.Usr, Test" table="Usr">
      <id name="Id" type="Int32" unsaved-value="null">
         <column name="UsrId" length="4" sql-type="int" not-null="true" unique="true" index="PK_Usr"/>
         <generator class="increment"/>
      </id>
      <property name="UsrCode" type="String">
         <column name="UsrCode" length="10" sql-type="varchar" not-null="true"/>
      </property>
      <many-to-one name="UpdatedBy" class="Test.Usr, Test" lazy="false">
         <column name="UpdatedBy" length="4" sql-type="int" not-null="false"/>
      </many-to-one>
   </class>
</hibernate-mapping>

Problem when I am modifying RequestedBy, the main class is also update. For example

Usr usr = session.Get<Usr>(1);

usr.UpdatedBy.Id = 2; <===== At this line, usr.Id is also updated to 2. It looks usr and UpdatedBy is pointing to the same Usr object (due to NHibernate caching?).

How can I tell NHibernate not to use cache? I've tried Evict(typeof(Usr)) after Get but it's too late. Should I explore IInterceptor? If yes any hint?

Thanks.

wolli · **Posted:** Mon Jul 21, 2008 4:00 am

Since you have an association to another object here, you can't change it by simly setting another id to the object. You have to replace the whole object:

usr.UpdatedBy = usr2;

As an alternative, you can just map the id of the user instead of the whole object. Then your approach would work.

jwira · **Joined:** Mon Jul 21, 2008 2:52 am **Posts:** 8

Thanks for your prompt reply.

However, the problem is that on this statement usr.UpdatedBy = usr2. It seems that NHibernate using the cached usr for both usr and usr.UpdatedBy objects since they both are usr.Id = 1.

How do I clear the cache or tell NHibernate to create a new usr object for UpdatedBy even though the id is the same?

wolli · **Posted:** Mon Jul 21, 2008 9:04 am

You can't have more than one object with the same id ! It's the primary key. From what you have described, it doesn't sound like a caching problem. Can you describe what you want to achieve in more detail ?

jwira · **Joined:** Mon Jul 21, 2008 2:52 am **Posts:** 8

Hi Here is the picture representation of the problem.

Code:

Retrieve Process for usr.id =1 with data as per first post

UsrId    UsrCode  UpdatedBy
 1         tuser1     1

  Populate Usr
+------------------+       +------------------+
|                  |       |                  |
|  usr.Id = 1      | <---- |      usr         |
|   (Cached)       |       |                  |
|                  |       |                  |
+------------------+       +------------------+

  Populate UpdatedBy

+------------------+       +------------------+
|                  |       |                  |
|  usr.Id = 1      | <---- |      usr         |
|   (Cached)       |       |                  |
|                  |       |                  |
+------------------+       +------------------+
               ^             /
               |            /
               \           /
                \         V
        +------------------+
        |                  |
        |   UpdatedBy      |
        |                  |
        |                  |
        +------------------+

Nhibernate cache noticed that usr and UpdatedBy is the same record and reuse the cached Usr object. As the result, when I change UpdatedBy.Id to 2, usr.Id is also changed to 2 as they usr and UpdatedBy are pointing to the same object.

I want This to happen

Code:

  Populate Usr
+------------------+       +------------------+
|                  |       |                  |
|  usr.Id = 1      | <---- |      usr         |
|                  |       |                  |
|                  |       |                  |
+------------------+       +------------------+


  Populate UpdatedBy


+------------------+       +------------------+
|                  |       |                  |
|  usr.Id = 1      | <---- |      usr         |
|                  |       |                  |
|                  |       |                  |
+------------------+       +------------------+
                                 | 
                                 |
                                 |
                                 v
+------------------+      +------------------+
|                  |       |                  |
|  usr.Id = 1      | <---- |    UpdatedBy     |
|                  |       |                  |
|                  |       |                  |
+------------------+       +------------------+

When I change UpdatedBy.Id to 2, usr.Id won’t be changed to 2 as they are 2 different objects.

wolli · **Posted:** Tue Jul 22, 2008 2:02 am

This is no caching problem. Hibernate is about objects, not values. Changing the id of an persisted object is not the same like changing the reference.

If you want to assign a different usr to UpdatedBy, you have to assign a different usr object, either by creating a new one (with id 2) or loading it from the database.

Code:

Usr usr1 = session.Get<Usr>(1); 
Usr usr2 = session.Get<Usr>(2); // or usr2 = new Usr(2);

usr1.UpdatedBy = usr2;

I assume you want UpdatedBy to be set to the current user to provide some kind of change information. I furthermore assume that you have the id or the current user available through some "manager" class. So at the end your code would look like this:

Code:

usr1.UpdatedBy = usrMgr.CurrentUsr;

I hope that makes it clearer.

jwira · **Joined:** Mon Jul 21, 2008 2:52 am **Posts:** 8

Hi Wolli,

Thanks for your effort to help me. I've tried your suggestion and still having the problem. Could you use my code and try it (if you have the time) then you'll have a better picture of my problem.

wolli · **Posted:** Wed Jul 23, 2008 2:55 am

Can you post the code of your last try ? Everthing that happens between opening the session and closing the session.

jwira · **Joined:** Mon Jul 21, 2008 2:52 am **Posts:** 8

Hi Wolli,

I've tried again using your suggestion

usr.UpdatedBy = usr2;

instead of
usr.UpdatedBy.id = usr2.id;

It's working now. I must have done something wrong.

Thanks for your help.