nHibernate.DuplicateMapping异常

我得到了一个 nHibernate.DuplicateMapping异常 我不明白。这个应用程序是一个简单的项目经理。它包含一个Project类,该类具有ProjectNote类型的Notes属性。以下是错误消息:

NHibernate.MappingException: Could not compile the mapping document: ProjectManager.Domain.Mapping.ProjectNote.hbm.xml ---> NHibernate.DuplicateMappingException: Duplicate class/entity mapping ProjectNote

类定义和映射文件如下所示。

有人能告诉我为什么会出现重复映射异常吗?谢谢。

using System.Collections.Generic;

namespace ProjectManager.Domain
{
    public class Project
    {
        #region Constructor

        public Project()
        {
            Initialize();
        }

        #endregion

        #region Properties

        /// <summary>
        /// The ID of this project.
        /// </summary>
        public virtual int ID { get; set; }

        /// <summary>
        /// The value used to sort this item in a Projects list.
        /// </summary>
        public virtual int Index { get; set; }

        /// <summary>
        /// The name of this project.
        /// </summary>
        public virtual string Name { get; set; }

        /// <summary>
        /// Notes for this project.
        /// </summary>
        public virtual IList<ProjectNote> Notes { get; set; }

        /// <summary>
        /// The tasks in this project.
        /// </summary>
        public virtual IList<Task> Tasks { get; set; }

        #endregion

        #region Private Methods

        /// <summary>
        /// Initializes this class.
        /// </summary>
        private void Initialize()
        {
            Tasks = new List<Task>();
            Notes = new List<ProjectNote>();
        }

        #endregion
    }
}

namespace ProjectManager.Domain
{
    public class ProjectNote
    {
        #region Properties

        /// <summary>
        /// 
        /// </summary>
        public virtual int ID { get; set; }

        /// <summary>
        /// The parent project of this note.
        /// </summary>
        public virtual Project Parent { get; set; }

        /// <summary>
        /// The text of the note
        /// </summary>
        public virtual string Text { get; set; }

        #endregion
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   auto-import="true"
                   assembly="ProjectManager.Domain"
                   namespace="ProjectManager.Domain">

  <!-- Map class 'Project' -->
  <class name="Project" table="Projects">

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

    <!-- Simple properties -->
    <property name="Name" column="Name" type="String" not-null="true" />

    <!-- Collection properties: Parent-side -->
    <bag name="Tasks" table="Tasks" cascade="all-delete-orphan" inverse="true">
      <key column="ProjectID" />
      <one-to-many class="Task" />
    </bag>

    <bag name="Notes" table="ProjectNotes" cascade="all-delete-orphan" inverse="true">
      <key column="ProjectID" />
      <one-to-many class="ProjectNote" />
    </bag>


  </class>

</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   auto-import="true"
                   assembly="ProjectManager.Domain"
                   namespace="ProjectManager.Domain">

  <!-- Map class 'ProjectNote' -->
  <class name="ProjectNote" table="ProjectNotes">

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

    <!-- Simple properties -->
    <property name="Text" column="Text" type="String" />

    <!-- Collection properties: Child-side -->
    <many-to-one name="Parent" column="ProjectID" class="Project" />

  </class>

</hibernate-mapping>