Binding中使用RelativeSource

问题：当前View有一个DataGrid，其DataGridTemplateColum设置了一个ComboBox。

其对应的VM是local:ViewMode，含有ModelValue和myLIst属性分别用来做DataGrid的ItemSource，和ComboBox的ItemSource。外层内层看起来链接的VM层不同的property。

初始代码如下：

<DataGrid ItemsSource="{Binding ModelValues, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" >
       <DataGridTemplateColumn Header="myHeader" Width="200">
            <DataGridTemplateColumn.CellTemplate>
                 <DataTemplate>
                      <ComboBox DisplayMemberPath="Value" SelectedValuePath="Key" IsEnabled="False"
                           SelectedValue="{Binding myID, Mode=TwoWay}"
                           ItemsSource="{Binding Path=myList,
                           RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:ViewModel}}}" />
                 </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
            <DataGridTemplateColumn.CellEditingTemplate>
                 <DataTemplate>
                      <ComboBox DisplayMemberPath="Value" SelectedValuePath="Key" IsEnabled="False"
                           SelectedValue="{Binding myID, Mode=TwoWay}"
                           ItemsSource="{Binding Path=myList,
                           RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:ViewModel}}}" />
                 </DataTemplate>
            </DataGridTemplateColumn.CellEditingTemplate>
       </DataGridTemplateColumn>
  </DataGrid>

错误在于RelativeSource沿着Visual Tree找不到一个叫local:ViewModel的图形对象

RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:ViewModel}}

解决代码：The Relativesource is for the Visual Tree, not the DataContext

<DataGrid ItemsSource="{Binding ModelValues, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" >
  <DataGridTemplateColumn Header="myHeader" Width="200">
     <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <ComboBox DisplayMemberPath="Value" SelectedValuePath="Key" IsEnabled="False"
                      SelectedValue="{Binding myID, Mode=TwoWay}"
                      ItemsSource="{Binding Path=DataContext.myList,
                      RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}}" />
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
    <DataGridTemplateColumn.CellEditingTemplate>
        <DataTemplate>
            <ComboBox DisplayMemberPath="Value" SelectedValuePath="Key" IsEnabled="False"
                      SelectedValue="{Binding myID, Mode=TwoWay}"
                      ItemsSource="{Binding Path=DataContext.myList,
                      RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}}" />
        </DataTemplate>
    </DataGridTemplateColumn.CellEditingTemplate>
  </DataGridTemplateColumn>
</DataGrid>