NotSupportedException when I'm using a method in the where clause

Question

I'm using nHibernate 3.2.0.4000. I wrote this query using nHibernate.Linq

var entities = (from t in this.Session.Query<Task>()
                where NotIn(t, role.Tasks)
                select t).ToList();

Here's the definition of the method NotIn()

private bool NotIn(Task t, IEnumerable<TaskDto> tasks)
{
    foreach (var task in tasks)
    {
        if (t.Name == task.Name) return false;
    }
    return true;
}

When I'm executing this query, I've got an NotSupportedException error:

Boolean NotIn(Probel.NDoctor.Domain.DAL.Entities.Task, System.Collections.Generic.IEnumerable`1[Probel.NDoctor.Domain.DTO.Objects.TaskDto])

I found a non Linq solution that is less readable but I still want to, at least, understand why it is impossible to build a Linq query like this.

Thank you in advance for your help!

Answer 1

You have to translate NotIn to nHibernate SQL query using expression tree.

nhibernate linq provider extension is a good starting point.

this link has In and NotIn extension method for nHibernate.

Answer 2

Your code in Linq is ultimately translated to SQL query by nhibernate. You can not use a method which could not be translated into SQL code.

Answer 3

NHibernate cannot decompile and then parse your code to get valid SQL. There is no way to generate an sql statement from your method.

Answer 4

Instead use:

var entities = (from t in this.Session.Query<Task>()
                where !role.Tasks.Any(rt => rt.Name == t.Name)
                select t).ToList();