Entity framework 如何在EntityDataSource中添加子表的计数

我有一个EntityDataSource，用于从tblOrderFile获取行数据，如下所示：

<asp:EntityDataSource ID="entityDataSourcePreorder" runat="server" 
        ConnectionString="name=iDBEntities" 
        DefaultContainerName="iDBEntities" EnableFlattening="False" 
        EntitySetName="tblOrderFiles" 
        Select="it.[pkOrderFileID], it.[fkOrderFileStatusID], it.[Filename], it.[CreateDate], it.[UserId]" 
        AutoGenerateWhereClause="True" EntityTypeFilter="" Where="">

    <asp:EntityDataSource ID="entityDataSourcePreorder" runat="server" 
        ConnectionString="name=iDBEntities" 
        DefaultContainerName="iDBEntities" EnableFlattening="False" 
        EntitySetName="tblOrderFiles" Include="tblOrderFileItems"
        AutoGenerateWhereClause="True" EntityTypeFilter="" Where="" >
    </asp:EntityDataSource>

现在我想修改它，以返回实体集名称为tblOrderFileItems的子表tblOrderFileItem中的行数

我找到了一种通过添加Include指令使计数正常工作的方法，如下所示：

<asp:EntityDataSource ID="entityDataSourcePreorder" runat="server" 
        ConnectionString="name=iDBEntities" 
        DefaultContainerName="iDBEntities" EnableFlattening="False" 
        EntitySetName="tblOrderFiles" 
        Select="it.[pkOrderFileID], it.[fkOrderFileStatusID], it.[Filename], it.[CreateDate], it.[UserId]" 
        AutoGenerateWhereClause="True" EntityTypeFilter="" Where="">

    <asp:EntityDataSource ID="entityDataSourcePreorder" runat="server" 
        ConnectionString="name=iDBEntities" 
        DefaultContainerName="iDBEntities" EnableFlattening="False" 
        EntitySetName="tblOrderFiles" Include="tblOrderFileItems"
        AutoGenerateWhereClause="True" EntityTypeFilter="" Where="" >
    </asp:EntityDataSource>

但我相信这是返回每个订单项的所有行的所有列。我只想要计数，不想将其余的数据传递到web页面

我还尝试简单地将it.tblOrderFileItems.Count添加到Select语句中，但得到一个错误，即

“Count”不是“Transient.collection[MyDBModel.tblOrderFileItemNullable=True，DefaultValue=]”的成员。要提取集合元素的属性，请使用子查询对集合进行迭代