NHibernate 'Category' Child/Parent mapping (XML)

Question

I'm trying to get NHibernate (3.3.1) to load a recursive parent/child category relationship.

public class Category 
{
    public virtual int Id { get; set; }

    public virtual bool IsActive { get; set; }

    public virtual string Name { get; set; }

    public virtual Category Parent { get; set; }

    public virtual IList<Category> Children { get; set; }

    public Category()
    {
        Children = new List<Category>();
    }

    public virtual int GetChildCount()
    {
        return Children.Count;
    }
}

And my XML Mapping...

 <class name="nHibernatePOC.Domain.Category, nHibernatePOC" lazy="true">
    <id name="Id" column="CategoryId">
      <generator class="identity" />
    </id>
    <property name="Name" column="Name" />
    <property name="IsActive" column="IsActive" />
    <many-to-one name="Parent" class="nHibernatePOC.Domain.Category" column="ParentCategoryId" /> 

    <bag lazy="true" name="Children">
      <key column="ParentCategoryId" />
      <one-to-many class="nHibernatePOC.Domain.Category" />
      <loader query-ref="GetCategoryByParentId"/>
    </bag> 
  </class>

My issue is when I try to access Children.Count I get a NullReferenceException because the Parent can be null.

Answer 1

For future reference, the problem was related to the stored procedure that was used to load the child collection. (Rather than lazy loading from a table / view). Thankfully I am in a position to load from a view instead of using a Stored Procedure.

The moment I removed the <loader element and reverted to using a view from the database (MSSQL 2012) it worked as expected.

Very odd, sorry to anyone who needs to use stored procedures but I'm guessing it's a good reason to remove them in this case.