流利的NHibernate子类分页具有多种类型

Question

I'm about to embark on a rather complicated mapping strategy and I have a sticking point that I'm not sure what the best track is...

Basically, I have a table per subclass schema with four subclasses. I'm using a standard repository pattern around NHibernate (for now) that this might not fit neatly into, which uses the LINQ provider (part of why I might change it later for caching and lazy loading stuff support from my mappings).

My requirement is that I must pull a list of TWO DIFFERENT subclasses at one time, so that I can implement paging over the query set. I am not sure how to do that...

I was thinking something along the lines of

Session.Query<Superclass>().Where(x => x is SubClass1 || x is SubClass2).Skip().Take();

But I'm unclear how that would translate, or if there is a better way?

Answer 1

Actually your example query is exactly how you would do it. Of course it depends on if you map your super and sub classes correctly with one ClassMap and several SubclassMaps (in fluent nhibernate).

You can query different sub types by using the is statement

var result = session.Query<LifeForm>().Where(t => t is Cat || t is Programmer).Skip(5).Take(10).ToList();

You can even do queries where you filter on sub type specific properties doing something like this:

var result2 = session.Query<LifeForm>()
    .Where(t => t is Cat || t is Programmer)
    .Where(p=>((p as Cat).Cuteness > 5) ||((p as Programmer).IsSenior == true)).ToList();

The downside of this is that nhibernate creates enormous queries left outer joining all sub types regardless of which sub types you are using and filters the results by heavily using case statements on the PK .

...
where 
case when csclifefor0_3_.Id is not null then 3 
when csclifefor0_2_.Id is not null then 2 
when csclifefor0_4_.Id is not null then 4 
when csclifefor0_5_.Id is not null then 5 
when csclifefor0_9_.Id is not null then 9 
when csclifefor0_10_.Id is not null then 10 
...

This might not be an issue though. The generated SQL statements just get more complex the more sub classes you might have...