MVVM How to bind command to ContextMenu - wpf

Question

I am completely confused using relativeSource and ancestorLevel.
Relative source is used for to get source from another elements. But to do that successfully you have to count at what level is that element. (How to debug?) It is most confusing part in WPF.
In my example i have context menu that i want to bind datasource and then command. How would binding must be to get command in my vm? Thank you
<Page.DataContext>
<PDB:UsersViewModel x:Name="vm"/>
</Page.DataContext>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<!--Page Header info content-->
<Grid Grid.Row="0">
<TextBlock Text="{Binding SelectedUser.Name}"/>
<TextBlock Text="{Binding ElementName=myGrd, Path=CurrentColumn.DisplayIndex}"/>
</Grid>
<!--Datagrid content-->
<DataGrid x:Name="myGrd"
SelectionMode="Single"
SelectionUnit="Cell"
CurrentItem="{Binding SelectedUser, Mode=TwoWay}"
CurrentColumn="{Binding CurrentColumn, Mode=TwoWay}"
IsReadOnly="True"
Grid.Row="1"
ItemsSource="{Binding FilteredUserList}"
AutoGenerateColumns="True"
CanUserAddRows="False"
>
<DataGrid.Resources>
<ContextMenu x:Key="ContextMenu">
<ContextMenu.Items>
<MenuItem Header="{Binding
RelativeSource={RelativeSource
FindAncestor,
AncestorType={x:Type Page},
AncestorLevel=4}, Path=vm}" />
</ContextMenu.Items>
</ContextMenu>
</DataGrid.Resources>
<DataGrid.CellStyle>
<Style TargetType="DataGridCell">
<Setter Property="ContextMenu" Value="{StaticResource ContextMenu}"/>
</Style>
</DataGrid.CellStyle>
</DataGrid>
</Grid>

Answer 1

You can't use RelativeSource in ContextMenu, because the menu is not a part of the visual tree. However this can be avoided by using Binding Source and x:Reference.
I assume your ViewModel looks like this
public class UserViewModel
{
public string Header { get; set; }
public ICommand MyCommand { get; }
... more code
}
Now let's bind Header and MyCommand properties of the VM
<ContextMenu x:Key="ContextMenu">
<ContextMenu.Items>
<MenuItem Header="{Binding Header, Source={x:Reference vm}}"
Command="{Binding MyCommand, Source={x:Reference vm}}"/>
</ContextMenu.Items>
</ContextMenu>
The important part is to have the ViewModel somewhere in the visual tree and set its x:Name, just like you've done in your example
<Page.DataContext>
<PDB:UsersViewModel x:Name="vm"/>
</Page.DataContext>
If you still want to know more about RelativeSource, this question seems to have the same problem as you. Basically Path of the binding has to be DataContext.MyViewModelProperty and the RelativeSource of the binding must be and element with DataContext set to the ViewModel.