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LanceVance
 Post subject: Newbie join Question
PostPosted: Sun Oct 02, 2005 3:13 am 
Beginner
Beginner

Joined: Sun Jul 31, 2005 1:23 pm
Posts: 35
Simplified version of the object structure at hand:

Code:
class A
{
   public int id;
   public B bReference;
};

class B
{
   public int id;
   public String someData;
};


My simplified code:

Code:
   Session session = HibernateUtil.getCurrentThreadSession();   
   try {   
      Query query = session.createQuery("from A as a");
      searchResults = query.list();
   } finally {
      HibernateUtil.closeSession();
   }


"searchResults" contains a collection of "A" instances (or proxy classes derived from "A"). However, when code tries to use the accessor to the bReference property, I get an exception:

javax.servlet.ServletException: javax.servlet.jsp.JspException: could not initialize proxy - the owning Session was closed

I assume this happens because the B instances weren't loaded and Hibernate is trying to load them when the accessor method gets called which is after the session is closed. Is this right?

I want Hibernate to preload the B instances and mappings as well. Does that make sense? How do I do that?

Or am I not supposed to close the session within a web app (JSF) and simply let each thread maintain an open session to Hibernate?

Thanks for any help!
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christian
 Post subject:
PostPosted: Sun Oct 02, 2005 6:58 am 
Hibernate Team
Hibernate Team

Joined: Mon Aug 25, 2003 9:11 pm
Posts: 4592
Location: Switzerland
This is very well explained in the documentation chapter "Working with objects".
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christian
 Post subject:
PostPosted: Sun Oct 02, 2005 7:01 am 
Hibernate Team
Hibernate Team

Joined: Mon Aug 25, 2003 9:11 pm
Posts: 4592
Location: Switzerland
Actually, its really an FAQ, it has several Wiki pages dealing with it, and the documentation chapter is

http://www.hibernate.org/hib_docs/v3/re ... ons-basics
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LanceVance
 Post subject:
PostPosted: Sun Oct 02, 2005 10:38 am 
Beginner
Beginner

Joined: Sun Jul 31, 2005 1:23 pm
Posts: 35
christian wrote:
Actually, its really an FAQ, it has several Wiki pages dealing with it, and the documentation chapter is

http://www.hibernate.org/hib_docs/v3/re ... ons-basics


I just reread that chapter; I see what it says regarding session use. It suggests one session per request and discusses using the HibernateUtil class and ThreadLocal which is what I am doing.

This doesn't really doesn't solve my problem. It doesn't say anything about how Hibernate loads many-to-one references and how I can force that loading during a query.
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LanceVance
 Post subject:
PostPosted: Sun Oct 02, 2005 11:05 am 
Beginner
Beginner

Joined: Sun Jul 31, 2005 1:23 pm
Posts: 35
OK, I resolved that first issue. Had to set lazy="false" in the mapping. So, I can control the auto-fetch at the mapping level but not at the query level. Now on to the next problem; this is a little trickier:

Simplified classes:

Code:
class A
{
   private int id;
   private B b;   // Ref to class b

   // basic getters/settters
};

class B
{
   private int id;
   private String someData;

   // basic getters/settters
};

class C extends B
{
   private String derivedClassData;

   // basic getters/settters
};


C.hbm.xml:

Code:
<joined-subclass
      name="package.C"
      table="C"
      extends="package.B">
   <key column="ID"/>

   <property name="derivedClassData" column="DerivedClassData" type="string" not-null="true"/>
</joined-subclass>


My simplified code:

Code:
   Session session = HibernateUtil.getCurrentThreadSession();   
   try {   
      Query query = session.createQuery("from A as a");
      searchResults = query.list();
   } finally {
      HibernateUtil.closeSession();
   }

   // Later in GUI framework, something like the following runs:
   for (Object o : searchResults) {
      A a = (A) a;
      B b = a.getB();   // Need to set lazy="false" in <key-many-to-one> in A.hbm.xml or else this will exception.

      // This will throw ClassCastException. Hibernate always returns direct instances of B
      // and never detects and creates the derived instances of C. Is there any way to get it
      // to do that?
      C c = (C) b;
   }
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