[hibernate-issues] [Hibernate-JIRA] Created: (HHH-5001) Polymorphism not always supported for queries and associations when using interface proxies

[Matt Accola (JIRA)]

Polymorphism not always supported for queries and associations when using interface proxies
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                 Key: HHH-5001
                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-5001
             Project: Hibernate Core
          Issue Type: Bug
          Components: core
    Affects Versions: 3.3.2
         Environment: Oracle 10
            Reporter: Matt Accola
         Attachments: HibernateInheritance.zip

On our project we have started using Hibernate inheritance widely and we are facing issues when we map entities in the inheritance hierarchies as lazy=true at the class level and when we map polymorphic, single-ended associations (many-to-one and one-to-one).  

Originally we were relying on the default Hibernate-generated proxies.  We were loading objects polymorphically using iterate or load.  In the HQL statement or the parameter to the load method we would specify the superclass type.  Also, for polymorphic, single-ended associations we were declaring the property to be of the superclass type.  The end result was when we try to downcast an object which should be a subclass into its subclass we get a ClassCastException.  

This behavior is well documented in section "19.1.3. Single-ended association proxies" of the Hibernate documentation.  In that section it describes a solution using proxy interfaces.  So, we created a parallel hierarchy of proxy interfaces for our inheritance hierarchy.  There are still issues with this approach.  I will summarize the most serious issues we have documented so far:

1. If the superclass is mapped as lazy=true and the subclass is mapped as lazy=true the subclass objects will not exhibit proper behavior when loaded polymorphically with iterate or load.  All possible interfaces are proxied, both correct interfaces and incorrect interfaces.  This makes instanceof operations unreliable.  The ClassCastException is simply deferred to the time when a method from the wrong subclass interface is invoked.

2. If the superclass is mapped as lazy=true and the subclass is mapped as lazy=true the subclass objects will not exhibit proper behavior when loaded via single-ended associations (many-to-one or one-to-one).  Same behavior as bullet point #1.

3. If the superclass is mapped as lazy=true and the subclass is mapped as lazy=false the subclass objects will not exhibit proper behavior when loaded polymorphically with iterate or load.  The object cannot be downcast to its subclass.

Changing all class-level mappings for all classes in the inheritance hierarchy to lazy=false AND changing all single-ended associations to eager fetch seems to resolve the issue.  However, we do not desire to eager fetch all these objects.

Another possible "fix" is to always use the exact subclass when loading the objects (in the HQL query or as a parameter to the load) but this eliminates polymorphism completely.  Also, it would be impossible to map polymorphic associations...all associations would need to declare a specific subclass.

I have uploaded an entire Eclipse project which demonstrates the issues with the proxy interface approach.  To run the tests in the project you need to do the following steps:

1. Import the project into Eclipse
2. Setup your DB connection properties in src/main/resources/local.properties
3. Run the script in DB.txt against your target schema

On a final note I spent a fair amount of time going through the JIRA issues that mentioned polymorphism.  This topic seems to have been touched several times and rejected quickly with a brief not to "read the documentation".  I assure you I have read the documentation many times searching for clues on how to accomplish this and I am at a dead end.  If there is a simple way to do this please take the time to provide a few sentences to lead us in the right direction.  Your help is appreciated!

Related JIRA issues?
http://opensource.atlassian.com/projects/hibernate/browse/HHH-2927
http://opensource.atlassian.com/projects/hibernate/browse/HHH-2921
http://opensource.atlassian.com/projects/hibernate/browse/HHH-1347
http://opensource.atlassian.com/projects/hibernate/browse/HHH-1288
http://opensource.atlassian.com/projects/hibernate/browse/HHH-4511
http://opensource.atlassian.com/projects/hibernate/browse/HB-382

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