Binding with Relativesource --wpf - wpf

Question

I have a short question:
I have a datagrid and binded it to something. Then I put a combobox in a column and binded it to another source by
ItemsSource="{Binding DataContext.Users, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}}"
Now I want to use datagrid's source again, in the combobox again.That code doesn't work now and I know that there is a ID column in datagrid's source.
SelectedValue="{Binding ID}"
How can I fix it? Thanks in advance.
And here is my whole DataGrid and the problematic part is the combobox.
<DataGrid ItemsSource="{Binding SCs}" SelectedItem="{Binding SelectedSC}" Margin="0,10,0,0"
RowStyle="{StaticResource ResourceKey=DataGridRowStyle}" Style="{DynamicResource ResourceKey=UILDataGridStyle}" AutoGenerateColumns="False" EnableRowVirtualization="True"
RowDetailsVisibilityMode="VisibleWhenSelected" Width="Auto" IsReadOnly="True" Background="Transparent"
VerticalAlignment="Stretch" HorizontalAlignment="Stretch" GridLinesVisibility="Vertical" VerticalGridLinesBrush="{DynamicResource ResourceKey=UILBorderBrush}"
RowHeaderWidth="0" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto" BorderBrush="{DynamicResource ResourceKey=UILBorderBrush}"
BorderThickness="1" SelectionUnit="FullRow" SelectionMode="Single" x:Name="SCDataGrid">
<DataGrid.Columns>
<DataGridTextColumn x:Name="securityConfigurationIDColumn" Binding="{Binding Path=SecurityConfigurationID}" Header="Security Configuration ID" Width="*" />
<DataGridTextColumn x:Name="securityConfigurationNameColumn" Binding="{Binding Path=SecurityConfigurationName}" Header="Security Configuration Name" Width="*" />
<DataGridTemplateColumn x:Name="proxyResponsibleUser_IDColumn" Header="Proxy Responsible User ID" Width="*" >
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<ComboBox IsEnabled="True" ItemsSource="{Binding DataContext.Users, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}}"
SelectedValuePath="{Binding UserID}" DisplayMemberPath="{Binding FullName}" Width="117" Height="20"
SelectedValue="{Binding ProxyResponsibleUser_ID, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" />
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
<DataGridTextColumn x:Name="responsibleUser_IDColumn" Binding="{Binding Path=User.FullName}" Header="Responsible User ID" Width="*" />
<DataGridTextColumn x:Name="securityConfigurationDescriptionColumn" Binding="{Binding Path=SecurityConfigurationDescription}" Header="Security Configuration Description" Width="*" />
<DataGridTextColumn x:Name="securityConfigurationURLColumn" Binding="{Binding Path=SecurityConfigurationURL}" Header="Security Configuration URL" Width="*" />
</DataGrid.Columns>
</DataGrid>

Answer 1

Use below cell template for combo box.
This code binds entire grid datasource to combo box template, which allow you use any property without doing the jugglery of finding ancestors..
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<Grid DataContext="{Binding }">
<ComboBox IsEnabled="True"
ItemsSource="{Binding Users}"
SelectedValuePath="{Binding UserID}"
DisplayMemberPath="{Binding FullName}"
Width="117" Height="20"
SelectedValue="{Binding ProxyResponsibleUser_ID,
UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"/>
</Grid>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>