I am trying to follow an NHibernate 1.2 HelloWorld application on VS2008/SQL 2005 RTM/NHibernate 2.1.x. I get a MappingException was unhandled error saying "No persister for:HelloWorldNHibernate.Employee". Googling suggested two probable causes for this problem. One, my Build Action for the hbm mapping file is not set to Embedded Resource (this I have set). Second, there's a syntax problem in the mapping file. But I can't figure out any problems in the mapping file as such. Any help is appreciated!

Employee.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
namespace="HelloWorldNHibernate"
assembly="HelloWorldNHibernate">
<class name="HelloWorldNHibernate.Employee" table="Employee">
<id name="ID" column="ID">
<generator class="identity"/>
</id>
<property name="Name" column="Name"/>
<many-to-one access="field" name="Manager" column="Manager" cascade="all"/>
</class>
</hibernate-mapping>

You probably have a problem with your hibernate configuration. There've been breaking changes between 1.2 and 2.0:

•<nhibernate> section is ignored, using <hibernate-configuration> section (note that they have different XML formats)
•Configuration values are no longer prefixed by "hibernate.", if before you would specify "hibernate.dialect", now you specify just "dialect"
•Will perform validation on all named queries at initialization time, and throw exception if any is not valid.

Can you post your configuration ?

Right.. I managed to get rid of the previous error by changing the way the mapping file was written. Now I get a "Could not compile the mapping document:HelloWorldNHibernate.Employee.hbm.xml" error. The inner exception says {"class HelloWorldNHibernate.Employee, HelloWorldNHibernate, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null not found while looking for property: Manager"}

Employee.hbm.xml:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
namespace="HelloWorldNHibernate"
assembly="HelloWorldNHibernate">
<class name="HelloWorldNHibernate.Employee, HelloWorldNHibernate" table="Employee">
<id name="ID" type="Int32" unsaved-value="0">
<generator class="native"/>
</id>
<property name="Name" column="Name" type="String" length="50"/>
<many-to-one access="field" name="Manager" column="Manager" cascade="all"/>
</class>
</hibernate-mapping>

hibernate.cfg.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
<session-factory>
<property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
<property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>
<property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
<property name="connection.connection_string">Server=localhost;Initial Catalog=HelloNHibernate;Integrated Security=SSPI</property>
<property name="proxyfactory.factory_class">NHibernate.ByteCode.Castle.ProxyFactoryFactory, NHibernate.ByteCode.Castle</property>
<property name="show_sql">true</property>
</session-factory>
</hibernate-configuration>

This is the Employee class:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace HelloWorldNHibernate
{
public class Employee
{
private int _id;
private string _name;
private Employee _manager;

public int ID
{
get { return _id;}
set { _id=value;}
}

public string Name
{
get { return _name; }
set { _name = value; }
}

public Employee Manager
{
get { return _manager;}
set { _manager = value; }
}

public string SayHello()
{
return string.Format("Hello World!', said {0}", Name);
}
}
}

Calling Program:

using System;
using System.Collections.Generic;
using System.Reflection;
using NHibernate;
using NHibernate.Cfg;

namespace HelloWorldNHibernate
{
class Program
{
static void Main(string[] args)
{
CreateEmployeeAndSaveToDatabase();
LoadEmployeeFromDatabase();

Console.WriteLine("Press <Enter> to exit");
Console.ReadLine();
}

static void CreateEmployeeAndSaveToDatabase()
{
Employee emp = new Employee();
emp.Name = "Sreedhar";

using (ISession session = OpenSession())
{
using (ITransaction transaction = session.BeginTransaction())
{
session.Save(emp);
transaction.Commit();
}
Console.WriteLine("Saved Employee to database");
}
}

static ISession OpenSession()
{
Configuration c = new Configuration();
//c.AddAssembly(Assembly.GetCallingAssembly());
c.Configure();
c.AddAssembly(typeof(Employee).Assembly);
ISessionFactory f = c.BuildSessionFactory();
return f.OpenSession();
}

static void LoadEmployeeFromDatabase()
{
using (ISession nhibernateSession = OpenSession())
{
IQuery query = nhibernateSession.CreateQuery("from Employee as emp order by emp.Name");
IList<Employee> foundEmployee = query.List<Employee>();
Console.WriteLine("\n{0} employees found", foundEmployee.Count);
foreach (Employee emp in foundEmployee)
{
Console.WriteLine(emp.SayHello());
}
}
}
}
}

<many-to-one access="field" name="Manager" column="Manager" cascade="all"/>

You're using field-based access, but your naming is wrong. You either have to change the field to

private Employee manager;

or change tha mapping to

field="field.camelcase-underscore"

Have a look at the tables at the end of that chapter:

http://nhforge.org/doc/nh/en/index.html#mapping-declaration-property

Thanx wolli this indeed was the problem! I'd just copied the original code, without realising. Thanks for that link. I removed the access attribute so it could default to property! I can see the SQL being generated. However, now I am getting an error since my ID field is not null in the database. The SQL is as follows:

NHibernate: INSERT INTO Employee (Name,Manager) VALUES (@p0, @p1); select SCOPE_IDENTITY;@p0 = 'Sreedhar', @p1 = NULL

What happened to my ID field? Why is it not getting picked up in the SQL?

You're using generator "native" which means the id will be generated in the database. Therefore no id is in the insert. You have to set the column to identity.

id name="ID" type="Int32" unsaved-value="0">
<generator class="native"/>
</id>

Brilliant :) !! I converted <generator class = "assigned"/> and assigned a value to the id in the calling program! Sweet success .. my first NHibernate now works like a charm. Thanks a ton wolli for all the assistance. Next step is to Update and then finally Delete the entry .. this is cool stuff!