Nhibernate映射一个具有泛型list属性的类

我是Nhibernate的新手，我正试图映射一个类(发票)，它具有使用Nhibernate的通用列表属性，但我不断得到错误:"从表InvoiceDetails关联到一个未映射的类:System.Int32"。因为每张发票都有更多的详细信息，所以我考虑了发票和InvoiceDetails表之间的一对多关系。

细节:a)发票类别:

public class Invoice
{
    private IList<InvoiceDetail> _invoiceDetails = new List<InvoiceDetail>();
    public virtual int InvoiceID { get; set; }
    public virtual string SerialNumber { get; set; }
    public virtual string Number { get; set; }
    public virtual DateTime InvoiceDate { get; set; }
    public virtual decimal Amount { get; set; }
    public virtual IList<InvoiceDetail> InvoiceDetails
    {
        get { return _invoiceDetails; }
        set { _invoiceDetails = value; }
    }
    public virtual Customer customer { get; set; }
    public virtual Institution Institution { get; set; }
    public virtual Receipt receipt { get; set; }
    public virtual bool IsDeleted { get; set; }
}

b) InvoiceDetail类:

public class InvoiceDetail
{
    public virtual int InvoiceDetailID { get; set; }
    public virtual int InvoiceID { get; set; }
    public virtual string ServiceDescription { get; set; }
    public virtual string Unit { get; set; }
    public virtual int Quantity { get; set; }
    public virtual double Value { get; set; }
    public virtual bool IsDeleted { get; set; }
}

c)发票映射文件:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
    assembly="MyProject" namespace="MyProject.Model">
    <class name="Invoice" table="Invoices">
        <id name="InvoiceID" column="InvoiceID" type="int">
            <generator class="native"></generator>
        </id>
        <property name="SerialNumber" column ="InvoiceSerialNumber"/>
        <property name="Number" column ="InvoiceNumber"/>
        <property name="InvoiceDate"/>
        <property name="Amount"/>
        <property name="IsDeleted" />
        <many-to-one name="Institution" column="InstitutionID" />
        <bag name="InvoiceDetails" access="nosetter.camelcase"
            inverse ="true" lazy ="false" cascade="all-delete-orphan">
            <key column="InvoiceID"/>
            <many-to-one class="MyProject.Model.InvoiceDetail, MyProject"/>
        </bag>
    </class>
</hibernate-mapping>

d) InvoiceDetail映射文件:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
    assembly="MyProject" namespace="MyProject.Model">
    <class name="InvoiceDetail" table="InvoiceDetails">
        <id name="InvoiceDetailID" column="InvoiceDetailID" type="int">
            <generator class="native"></generator>
         </id>
         <many-to-one name="InvoiceID" column="InvoiceID" />
         <property name="ServiceDescription"/>
         <property name="Unit"/>
         <property name="Quantity"/>
         <property name="Value"/>
         <property name="IsDeleted" />    
     </class>
 </hibernate-mapping>

public virtual int InvoiceID { get; set; }

public virtual Invoice Invoice { get; set; }

<many-to-one name="InvoiceID" column="InvoiceID" />

<many-to-one name="Invoice" column="InvoiceID" />

<bag name="InvoiceDetails" access="nosetter.camelcase" inverse ="true" lazy ="false" cascade="all-delete-orphan">
  <key column="InvoiceID"/>
  <many-to-one class="MyProject.Model.InvoiceDetail, MyProject"/>
</bag>

<bag name="InvoiceDetails" access="nosetter.camelcase" inverse ="true" lazy ="false" cascade="all-delete-orphan">
  <key column="InvoiceID"/>
  <one-to-many class="MyProject.Model.InvoiceDetail, MyProject"/>
</bag>