Error while conversion of Nhibernate query to generic list

Question

I have a simple entity called EmployeeEntity with properties ID, Name, Age, Organisation, and Designation. I am just querying the database using the query

IQuery query = session.CreateQuery(
    "select Name, Designation, Age, Organisation FROM EmployeeEntity " +
    "group by Name, Designation, Age, Organisation");

IList<EmployeeEntity> employee = query.List<EmployeeEntity>(); // Throws error

but on conversion to my type, it's throwing an exception:

Could not execute query[SQL: SQL not available]

with InnerException:

The value "System.Object[]" is not of type "NHibernateTest.EmployeeEntity" and cannot be used in this generic collection.
Parameter name: value

though it works fine using this query:

IQuery query = session.CreateQuery("select e FROM EmployeeEntity e group by e");

IList<EmployeeEntity> employee = query.List<EmployeeEntity>();

but I don't want to select all the columns because I don't need them.

Answer 1

If you only want a certain set of columns, create a class that maps one to one with your columns. Like so:

public class EmployeeView
{
    public string Name { get; set; }
    public string Designation { get; set; }
    public int Age { get; set; }
    public string Organization { get; set; }
}

You then just need to add a result transformer to your query

IQuery query = session
    .CreateQuery("select Name ,Designation ,Age ,Organisation  FROM EmployeeEntity   group by  Name ,Designation ,Age ,Organisation")
    .SetResultTransformer(Transformers.AliasToBean<EmployeeView>());

Ilist<EmployeeEntity> employee= query.List<EmployeeView>();

Answer 2

When you're querying with select Name, Designation, Age, Organisation..., NHibernate will actually return an IList<object[]> instance. To overcome this, try rewriting your HQL to select new EmployeeEntity(Name, Designation, Age, Organisation)... and add an appropriate constructor to EmployeeEntity class.