Customization of nhibernate <set> querying - nhibernate

Question

I want to get ClassA.ClassBCollection property filtered and paged. I need to change filtering dynamically.
The default querying will result in something like:
select * from ClassA
left outer join ClassB
on id == FK_ClassB
Can I customize querying of nhibernate set somehow?
Mappings:
<class name="ClassA">
<property name="Name" />
<set name="ClassBCollection">
<key column="FK_ClassB" on-delete="cascade" />
<one-to-many class="ClassB" />
</set>
</class>
<class name="ClassB">
<property name="Something"/>
</class>

Answer 1

If I do understand your question...
Can I customize querying of nhibernate set somehow?
...correctly, the answer is NO.
I mean, if you think about getting the instance of ClassA and doing some paging and filtering over its <set> collection. That would be always done in memory. (What we can do with mapping I appended at the end).
we can change the approach
In this case, when you need a filter and paging over the collection items, I would strongly recommend to go the other way. Create Criteria (QueryOver, HQL) not over the ClassA but over the ClassB.
First of all we have to extend ClassB mapping:
<class name="ClassB">
<property name="Something" />
<many-to-one name="ClassA" column="FK_ClassB" fetch="join" />
</class>
And then create a Criteria like this
var criteria = NHSession.Current.CreateCriteria<ClassB>();
criteria
.Add(new InExpression("ClassA", new object[] {1})) // id of one or more ClassA
.AddOrder(new Order("Something", true)) // Order By
.SetFirstResult(2) // Skip
.SetMaxResults(10); // Take
var list = criteria.List<ClassB>();
Because we used mapping of ClassA fetch="join" the resulting SQL statement will be very similar to the first snippet in this question.
So this way, we can achieve the desired SQL Select, but we cannot use ClassA.ClassBCollection directly. We did it this way...
NOTE:
Filters / paging which we can influence on the <set> mapping are static filter in the where clause and style of fetching values.
Where clause will always be evaluated when loading ClassBCollection as a property of the ClassA. It could be like where="IsActive=true"
In case that ClassA can have a lot of items in ClassBCollection, we can manage how they will be loaed. Very effective way is attribute batch-size documented here

Answer 2

Can I customize querying of nhibernate set somehow?
I'm not entirely sure what this means. If you meant, can I query and use WHERE clauses over the collection, the answer is yes. Here's how:
[TestFixture]
public class StackOverflowQuestion13496270Tests
{
public ISession session;
[SetUp]
public void SetUp()
{
session = // Get the current NHibernate session
}
[Test]
public void Query_ClassA()
{
var results = session.Query<ClassA>()
.Where( x => x.ClassBCollection.Any( y => y.Name == "Bob" ) )
.Fetch( x => x.ClassBCollection )
.Skip( 0 )
.Take( 50 )
.ToList();
}
[Test]
public void Query_ClassB()
{
var results = session.Query<ClassB>()
.Where( x => x.Name == "Bob" )
.Fetch( x => x.ClassAParent )
.Skip( 0 )
.Take( 50 )
.ToList();
}
public class ClassA
{
public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual IList<ClassB> ClassBCollection { get; set; }
}
public class ClassB
{
public virtual int Id { get; set; }
public virtual string Name { get; set; }
// Add this and the appropriate mapping modifications to be able to navigate back to the parent
public virtual ClassA ClassAParent { get; set; }
}
}