Conditional Order By Clause in HQL

2009-07-09T01:00:00.000-07:00

For reasons I have not yet found, a query including an order by statement like below does not seem to work:

"From Rate R 
                    Where (R.EndEffectiveDate Is Null And R.BeginEffectiveDate < :oldDate) 
                            Or 
                           R.EndEffectiveDate < :oldDate
                           And
                           R.Zone = :zone And R.PriceList.Supplier = :supplier And R.PriceList.Customer = :customer
                    ORDER BY 
                        (Case When R.EndEffectiveDate Is Null then R.BeginEffectiveDate else R.EndEffectiveDate end) desc"

NHibernate in Action, Working with persistent objects, Using business key equality

2008-10-23T20:27:00.000-07:00

This is one of a series of posts I will be doing as notes, ideas, questions and comments on content from the book NHibernate in Action.
While explaining the importance of a Business Key and how business key equality should be implemented in a class by overriding the Equals method of the Object class, the author mentions the following.

Quote from book:

"Notice that it’s almost never correct to
override Equals() on a subclass and include another property in the comparison. It’s tricky to satisfy the
requirements that equality be both symmetric and transitive in this case; and, more important, the business key
wouldn’t correspond to any well-defined candidate natural key in the database (subclass properties may be
mapped to a different table)."

This is not clear especially if we consider the following:

it would be tricky for equality to be both symmetric and transitive only if we are comparing (using Equals) an object of the base class with an object from the subclass. That does not seem like a thing that would usually be done since the base class is usually abstract. Take for example the example mentioned in the book: Base Class is "BillingDetails", subclasses are "CreditCard" and "BankAccount".
If the Base Type defines an implementation for Equals (by overriding Object.Equals), and the Subtype also defines its implementation of Equals (by overriding the implementation of BaseType), and we are referencing an object of the Subtype using a reference of the Base Type, then the Base Type Equals method will NOT be called. So the above argument does not hold if we are considering something like:

SubType obj1 = new SubType();

//some code to set the obj1 to a certain state
...

SubType obj2 = new SubType();

//some code to set the obj2 to same state as obj1
...

BaseType ref1 = obj1;

//the following statements are equivalent
ref1.Equals(obj2);
obj2.Equals(ref1);

So the above statements where Equals is called are equivalent.

I can understand how "the business key wouldn’t correspond to any well-defined candidate natural key in the database", it is just there for completeness.

So I am not sure what exactly the author meant, does he mean the specific case when we are comparing objects of different classes (one of the base type and another of a sub-type)? If so then maybe he should have provided such a constraint in his text.

Hello World!

2008-10-10T17:03:00.000-07:00

Welcome. This blog is intended as a repository of information related to NHibernate. Anyone interested in contributing, please contact me either by email (muhammadshehabeddeen@gmail.com) or here by commenting.
To kick it off, I will be posting my notes on the book "NHibernate in Action" (draft) as I read through it. I have a few notes ready, so expect another post soon.

NHibernation

Conditional Order By Clause in HQL

NHibernate in Action, Working with persistent objects, Using business key equality

Hello World!