namespace Timesheet.BusinessObjects
{
    /// <summary>
    /// Creates an instance of Customer.
    /// </summary>
    public class Customer : PersistentManager
    {
        // Properties
        public DateTime DateModified { get; private set; }
        public String CustomerName { get; private set; }
        public Int64 CustomerID { get; private set; }
        public Int64 ProjectID { get; private set; }

        // Constructor
        public Customer()
        {           
        }

        // Methods
        public static Customer CreateCustomer(string customerName)
        {
            Customer customer = new Customer();
            customer.CustomerName = customerName;
            customer.DateModified = DateTime.Now;
            customer.ProjectID = 0;
            customer.Save(); //Error here
            return customer;
        }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
  <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
    <class name="Timesheet.BusinessObjects.Customer" table="Customers" lazy="false">
      <id name="CustomerID">
        <column name="CustomerID" />
        <generator class="native" />
      </id>
      <property name="DateModified"></property>
      <property name="CustomerName"></property>
      <property name="CustomerID"></property>
      <property name="ProjectID"></property>
    </class>   
  </hibernate-mapping>