如何在网格单元格模板中的RadRibbonDropDownButton中绑定ListView

我有一个rad网格视图，它将itemssource绑定到一个'Employee's列表。'Employee'实体有一个'Roles'属性列表。我如何将这个"角色"绑定到ListView，如下所示。

为例:在第二列中，RadComboBox正确结合。在第三列，RadRibbonDropDownButton不能正常结合。我如何得到角色列表绑定到RadRibbonDropDownButton内的ListView ?

请帮助。

class Employee
{
    public string Name {get; set;}
    public List<Role> RolesList {get; set;}
}
class Role
{
    public int RoleId {get; set;}
    public string RoleName {get; set;}
    public string RoleCategory {get; set;}
}
<tk:RadGridView ItemsSource="{Binding Path=EmployeeList,Mode=TwoWay}" AutoGenerateColumns="False">
    <tk:RadGridView.Columns>
        <tk:GridViewDataColumn Header="Employee Name" DataMemberBinding="{Binding Name}" IsReadOnly="True"/>
        <tk:GridViewDataColumn Header="Roles" Width="410">
            <tk:GridViewDataColumn.CellTemplate>
                <DataTemplate>
                    <!--this binds properly-->
                    <tk:RadComboBox ItemsSource="{Binding RolesList}" DisplayMemberPath="RoleName" SelectedValuePath="RoleId" />
                </DataTemplate>
            </tk:GridViewDataColumn.CellTemplate>
        </tk:GridViewDataColumn>
        <tk:GridViewDataColumn Header="Roles" Width="410">
            <tk:GridViewDataColumn.CellTemplate>
                <DataTemplate>
                    <tk:RadRibbonDropDownButton>
                        <tk:RadRibbonDropDownButton.DropDownContent>
                            <ListView ItemsSource="{Binding RolesList}">
                                <ListView.View>
                                    <GridView AllowsColumnReorder="False">
                                        <GridViewColumn DisplayMemberBinding="{Binding RoleName}" Header="Role Name" Width="75" ></GridViewColumn>
                                        <GridViewColumn DisplayMemberBinding="{Binding RoleCategory}" Header="Role Category" Width="175" ></GridViewColumn>
                                    </GridView>
                                </ListView.View>
                            </ListView>
                        </tk:RadRibbonDropDownButton.DropDownContent>
                    </tk:RadRibbonDropDownButton>
                </DataTemplate>
            </tk:GridViewDataColumn.CellTemplate>
        </tk:GridViewDataColumn>
    </tk:RadGridView.Columns>
</tk:RadGridView>