Nhibernate - Query on entity A does not return related entity's B subclasses data

Question

I have an class called WorkflowTask that contains a TaskProperties class property. There is a number of subclasses for TaskProperties mapped using JoinedSubclassMapping (such as DeliveryTaskProperties).

I need to execute a NHibernate Query such as the following to get a List of the workflow tasks that I need to handle:

NHibernateSession.Query<WorkflowTask>().Where(x => (x.WorkflowTaskStatus ==  WorkflowTask.WorkflowTaskStatuses.New && x.TaskProperties.UserAssignedTo=="System")).ToList<WorkflowTask>();

When iterating through this list I notice that the correct subclass of TaskProperties is identified. However when I try to cast the wt.TaskProperties to DeliveryTaskProperties to access its data I get an invalid cast exception. The only way I can get this to work is to try to load the task properties entity again by specifying the exact subclass:

---

The following works and I can cast to the DeliveryTaskProperties class:

wt.TaskProperties = NHibernateSession.Load<DeliveryTaskProperties>(wt.TaskProperties.Id);

Immediate Window:

{Workflow.Entities.DeliveryTaskProperties}

[DeliveryTaskPropertiesProxy]: {Workflow.Entities.DeliveryTaskProperties}

base: {Workflow.Entities.DeliveryTaskProperties}

UserAssignedTo: "system"

---

This doesn't Work and I get an invalid cast exception:

wt.TaskProperties = NHibernateSession.Load<TaskProperties>(wt.TaskProperties.Id);

Immediate Window:

{Workflow.Entities.DeliveryTaskProperties}

[TaskPropertiesProxy]: {Workflow.Entities.DeliveryTaskProperties}

base: {Workflow.Entities.DeliveryTaskProperties}

UserAssignedTo: "system"

---

Classes Mapping:

     public class WorkflowTaskMapping : BaseWorkflowEntityMapping<WorkflowTask>
{
    public WorkflowTaskMapping()
    {

        this.Property(x => x.TaskPropertiesId, map =>
        {
            map.Column("TaskPropertiesId");
            map.Insert(false);
            map.Update(false);
            map.NotNullable(true);
        });

        this.ManyToOne<TaskProperties>(x => x.TaskProperties, map =>
        {
            map.Column("TaskPropertiesId");
            map.Cascade(Cascade.All);
            map.NotNullable(true);
            map.ForeignKey("WFTaskProperties_WFTask_FK");
        });

        this.Property(x => x.WorkflowTaskStatus, map => map.NotNullable(true));
    }
}

---

        public class TaskPropertiesMapping : BaseWorkflowEntityMapping<TaskProperties>
{
    public TaskPropertiesMapping()
    {
        this.Property(x => x.UserAssignedTo, map => map.NotNullable(true));
    }
}

---

        public class DeliveryTaskPropertiesMapping : JoinedSubclassMapping<DeliveryTaskProperties>
{
    public DeliveryTaskPropertiesMapping()
    {

        this.Key(x => { x.Column("Id"); x.ForeignKey("DelivTask_TaskProperties_FK"); });
        this.Property(x => x.DeliveryAddress, map => map.NotNullable(true));
        this.Property(x => x.Deadline, map => map.NotNullable(true));
        this.Property(x => x.DeliveryOnDeadline, map => map.NotNullable(true));

    }

}

---

Is there a way to load the subclass data correctly when loading the WorkflowTask entity and without having to specify the exact subclass type?

Answer 1

OK, I figured it out. Although it wasn't very clear on other posts I read dealing with similar issues it seems that Eager Fetching did the trick for me.

If I change my query and add fetching like this:

               NHibernateSession.Query<WorkflowTask>().Where(x => (x.WorkflowTaskStatus == WorkflowTask.WorkflowTaskStatuses.New && x.TaskProperties.UserAssignedTo == "System")).Fetch(x => x.TaskProperties).ToList<WorkflowTask>();

Then nhibernate actually loads the subclass in memory although the type remains that of the base class. This allows for later casting to the subclass to access its properties.

Answer 2

Im not sure if is this what you need:

TaskProperties tpAlias = null;

var workflowTasks = NHibernateSession.QueryOver<WorkflowTask>()
    .Left.JoinAlias(x => x.TaskProperties, () => tpAlias)
    .Where(x => (x.WorkflowTaskStatus ==  WorkflowTask.WorkflowTaskStatuses.New 
           && x.TaskProperties.UserAssignedTo == "System"))
    .List();