Here is aother strange problem related with NHibernate

The Problem

I have a Parent class and two child classes Child1 and Child2 mapped to different tables on the database.
Lets assume that we specified cascade='all' for child bags defined on Parent.hbm.xml. Sample workflow of instantiating parent and child objects is as the following

- Create a parent object.
- Insert 2 Child1 instances to child1 bag
- Insert 3 Child2 instances to child2 bag.
- Flush the session
- Refresh parent object
- We get 6 instances for each child bag (child1 and child2). But we expect 2 Child1 instances in child1 bag and 3 Child2 instances in child2 bag.

Ther problem is : NHibernate performs left outer join on Child1 and Child2 tables when Refresh is called for the parent object. This is unaccaptable, I think NHibernate should initialize child collections with seperate selects commited to the database, or may be distinguish the duplicated child instances automatically in the collections. (using idbag instead of bag is not an option)

Download the test case

• NH1472.rar (1,12 mb)

Requirements

•  NHibernate.Burrow

Watch This

•  NH-1472

I discovered this problem while developing a new system with the latest NHibernate.Burrow distribution (which in turn uses the latest NHibernate distribution). I spent some time Googling around to check if anyboy else met the same problem and found some entries but none of them specified exactly why this failure was happening and how we can solve this problem.

The Problem

My code simply executes the following steps

1. Call a factory method
2. Create a Curve
   
3. Create CurveInterval objects
   
4. Put CurveInterval objects inside the Curve.Intervals collection
   
5. Call the same factory function to create another Curve instance
6. Before creating Curve instance I perform some checks with HQL , this causes session to be flushed automaticaly
   
7. I get the error when session is about to be flushed automatically
   

XML-Code: Mapping file for the Curve class

001 <?xml version='1.0' encoding='utf-8'?> 
002<hibernate-mapping xmlns='urn:nhibernate-mapping-2.2'> 
003<class 
004name='GradeEntry.Domain.Curve, GradeEntry.Domain' 
005      table= 'LetterGrade'> 
006<id name='Id' 
007column='LG_ID' 
008unsaved-value='0'> 
009<generator class='native'/> 
010</id> 
011
012<version name='ManagedVersion' column='NhVersion' type="System.Int64" unsaved-value="0"/> 
013
014<property name='DateCreated'/> 
015<property name='CreatedByPersonId'/> 
016<property name='Locked'/> 
017<property name='LockChangedById' column ='LockChangedByID'/> 
018<property name='LockChangedOn'/> 
019<property name='LockMessage'/> 
020<property name='LogIdx'/> 
021<property name='LogCnt'/> 
022<property name='IsStable'/> 
023
024<bag name='Intervals' 
025order-by='LetterID asc' 
026table='LetterGradeList' 
027access='field.camelcase-underscore' 
028inverse='true' 
029cascade='delete-orphan, save-update' 
030lazy='true'> 
031<key column='LG_ID'/> 
032<one-to-many class='GradeEntry.Domain.CurveInterval, GradeEntry.Domain' /> 
033</bag> 
034
035<bag name='Log' 
036order-by='Idx asc' 
037table='LetterGradeLog' 
038access='field.camelcase-underscore' 
039inverse='true' 
040cascade='delete-orphan,save-update' 
041lazy='true'> 
042<key column='LG_ID'/> 
043<one-to-many class='GradeEntry.Domain.CurveLog, GradeEntry.Domain' /> 
044</bag> 
045
046</class> 
047</hibernate-mapping>
048
049
050

First Attempt To Reporoduce The Problem

I have prepared a sample project but I was not able to recreate the bug, and I know it is hard to tell what is the problem under these conditions. But I have some more findings about this issue.

- When I replace cascade of the bags to 'all-delete-orphan' from 'delete-orphan,save-update' everything is ok
- When I replace cascade of the bags to 'all, delete-orphan' from 'all-delete-orphan' I still get the assertion failure.

I think the problem is with the new implementation of cascade which is intended to support comma seperated list of cascade options.

Second Attempt To Reporoduce The Problem

After spending some more time trying to diagonise the problem I finally succeeded to reprouce the problem with atest case. I attached the test case.

Simply the problem resolves to something like this.

NOTE: All classes utilize managed versioning by defining ManagedVersion.

1. I create 2 parent objects save them , flush the session and refresh them
2. Then for each parent object I create some child objects of type Child1
3. I perform HQL to get all Child3 instances. Child3 class does not have any association or cascade relation with Parent, Child1, Child2 and ParentRef classes. The HQL I performed causes the session to auto flush thus inserting Child1 instances to the database. But somehow while auto flushing the session versions of the parent objects are not updated. I expect two updates to be submitted to the database for parent objects since we associated Child1 instances with the parents and that must cause a version increment on parent objects.
4. Then I try to perform another HQL query to get list of ParentRef objects and I get assertion failure. Note that ParentRef class is associated with Parent class which in turn defines cascade for Children1 and Children2 collections.
   

If we do not perform the HQL in step 3 Child1 instances will be inserted to the database and parent objects will be updated while performing the HQL in step 4 and we will not get the assertion failure.

I covered both cases in the attached test case.

Database scripts are included under DBScript folder.

Download test case

• NH1469.rar (1,12 mb)

Requirements

• NHibernate.Burrow

Watch This

• NHibernate Jira Issue (NH-1469)

See Also

• NH-1419
• NHMA-19
  

GoF book says that "Observer pattern should defina a one-to-many dependency between objects so that when one object changes state, all its dependenst are notified and updated automatically". Subscribing to RSS feeds is a nice analogy.  You subscribe to RSS feeds to show interest and you become an observer who demand for notification and RSS feeds become the subject and are responsable for providing information to all subscribers. I think this bit of information describing the pattern is enough, now lets see how we implement observer pattern.

Lets assume that we are developing a car controller software. One of the requirements is "Provide visual warning to the driver if seat belts are not locked". To meet the requirement we design

• SeatBelt class which is the subject. This class should notify interested objects about the state changes
• SeatBeltMonitor class is our observer. This class monitors for status change messages provided by the SeatBelt and displays messages

Implementing Observer Pattern in conventional way 

Download GoF_Patterns_Observer1.rar (20,36 kb)

It is suggested to specify an interface or abstract classes when dealing with patterns so that the objects we create will adhere to some predefined contract. So we start our implementation by specifying two interfaces ISubject and IObserver

C#-Code: Our interfaces

001    public interface ISubject
002    {
003        void RegisterObserver(IObserver observer);
004        void RemoveObserver(IObserver observer);
005    }
006
007    public interface IObserver
008    {
009        void Update(bool locked);
010    }

As I've mentioned above our SeatBelt class is the subject (source) of the state change and should implement the contract so that interested parties shall define their interest to the state changes More...

I believe Factory Pattern is one of the most known pattern in development community. Simply Factory Pattern states that we have a factory object which is used to create other objects. But GoF specification for the Factory Pattern is a little bit different. GoF book says that "We should define an interface for creating an object, but let subclasses decide which class to instantiate." 

How you implement the Factory Pattern depends on your custom needs. 1) If you want to control object instantiation behaviour of your factory provide a static class with static methods or sealed factory class to your clients, else 2) if you want to keep your code closed for modification but open for extension follow the GoF way by providing an interface or abstract factory class.

Method 1

Download GoF_Patterns_Factory1.rar (18,45 kb)

Now lets implement Factory Pattern for our Vehicle instantiation process so that we do not want our clients to have alternative factory implementation at all.

C#-Code: Abstract Vehicle superclass and some concrete implementations

001   //Abstract Vehicle class
002    public abstract class Vehicle
003    {
004        public abstract string Description { get; }
005    }
006    
007    //Concrete implementation
008    public sealed class Car : Vehicle
009    {
010        internal Car() { }
011        public override string Description
012        {
013            get { return "Car";}
014        }
015    }
016
017    //Concrete implementation
018    public sealed class Helicopter : Vehicle
019    {
020        internal Helicopter() { }
021        public override string Description
022        {
023            get { return "Helicopter"; }
024        }
025    }
026
027    //Concrete implementation
028    public sealed class Jet : Vehicle
029    {
030        internal Jet() { }
031        public override string Description
032        {
033            get { return "Jet"; }
034        }
035    }

I've chosen to implement my factory More...