Bound Command not firing on another viewModel? What Am I doing wrong?

Question

Hi

I cannot seem to bind a command to a button.I have a treeview on the left showing

Country 
  City etc..          

And I tabcontrol on the right. do I This uses 4 viewModels

rootviewModel-ContinentViewModel-CountryViewModel-CityViewModel

What I am building is based on

http://www.codeproject.com/KB/WPF/TreeViewWithViewModel.aspx

Now on one of the tabs I have a Toolbar with a button "TestButton" that I have mapped in zaml. This does not fire!

The reason is not firing is because I m binding the RootViewModel but the command that is bound in zaml is in the cityViewModel.

How Do I pass the datacontext from one view to the other? or how do I make the button fire. I need the command to be in the cityViewModel.

in output i get "BindingExpression path error: 'TestCommand' property not found on 'object"

Any Suggestions on how I bind it?

View "WorldExplorerView" where I bind the main DataContext

    public partial class WorldExplorerView
    {
        public WorldExplorerView()
        {
            InitializeComponent();
            var continents = Database.GetContinents();
            var rootViewModel = new RootViewModel(continents);
            DataContext = rootViewModel;
        }           
    }

CityViewModel

 public class CityViewModel : TreeViewItemViewModel
 {
    private  City _city;
    private RelayCommand _testCommand;
    public CityViewModel(City city, CountryViewModel countryViewModel):base(countryViewModel,false)
    {
        _city = city;
    }

    Properties etc......


    public ICommand TestCommand
    {
        get
        {
            if(_testCommand==null)
            {
                _testCommand = new RelayCommand(param => GetTestCommand(), param => CanCallTestCommand);
                ;
            }
            return _testCommand;
        }           
    }

    protected bool CanCallTestCommand
    {
        get { return true; }
    }

    private static void GetTestCommand()
    {
        MessageBox.Show("It works");
    }
}

ZAML

    <DockPanel>
    <DockPanel LastChildFill="True">
        <Label DockPanel.Dock="top" Content="Title " HorizontalAlignment="Center"></Label>
        <StatusBar DockPanel.Dock="Bottom">
            <StatusBarItem Content="Status Bar" ></StatusBarItem>
        </StatusBar>
        <Grid DockPanel.Dock="Top">
            <Grid.ColumnDefinitions>
                <ColumnDefinition/>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="2*"/>
            </Grid.ColumnDefinitions>
            <TreeView Name="tree" ItemsSource="{Binding Continents}">
                <TreeView.ItemContainerStyle>
                    <Style TargetType="{x:Type TreeViewItem}">
                        <Setter Property="IsExpanded" Value="{Binding IsExpanded,Mode=TwoWay}"/>
                        <Setter Property="IsSelected" Value="{Binding IsSelected,Mode=TwoWay}"/>
                        <Setter Property="FontWeight" Value="Normal"/>
                        <Style.Triggers>
                            <Trigger Property="IsSelected" Value="True">
                                <Setter Property="FontWeight" Value="Bold"></Setter>
                            </Trigger>
                        </Style.Triggers>
                    </Style>
                </TreeView.ItemContainerStyle>

                <TreeView.Resources>
                    <HierarchicalDataTemplate DataType="{x:Type ViewModels:ContinentViewModel}"
                                      ItemsSource="{Binding Children}">
                        <StackPanel Orientation="Horizontal">
                            <Image Width="16" Height="16" Margin="3,0" Source="Images\Continent.png"/>
                            <TextBlock Text="{Binding ContinentName}"/>
                        </StackPanel>
                    </HierarchicalDataTemplate>

                    <HierarchicalDataTemplate DataType="{x:Type ViewModels:CountryViewModel}"
                                      ItemsSource="{Binding Children}">
                        <StackPanel Orientation="Horizontal">
                            <Image Width="16" Height="16" Margin="3,0" Source="Images\Country.png"/>
                            <TextBlock Text="{Binding CountryName}"/>
                        </StackPanel>
                    </HierarchicalDataTemplate>
                    <DataTemplate DataType="{x:Type ViewModels:CityViewModel}" >
                        <StackPanel Orientation="Horizontal">
                            <Image Width="16" Height="16" Margin="3,0" Source="Images\City.png"/>
                            <TextBlock Text="{Binding CityName}"/>
                        </StackPanel>
                    </DataTemplate>
                </TreeView.Resources>
            </TreeView>
            <GridSplitter Grid.Row="0" Grid.Column="1" Background="LightGray"
                 Width="5" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>
            <Grid Grid.Column="2" Margin="5" >
                <TabControl>
                    <TabItem Header="Demo">
                        <DockPanel LastChildFill="True">
                           <ToolBar DockPanel.Dock="Top">
                               <!-- DOES NOT WORK-->
                                <Button Name="btnTest" Command="{Binding TestCommand}"  Content="Press me see if works"></Button>
                        </ToolBar> 
                            <TextBox></TextBox>
                        </DockPanel>

                    </TabItem>
                    <TabItem Header="Details" DataContext="{Binding Path=SelectedItem.City, ElementName=tree, Mode=OneWay}">
                        <StackPanel >
                            <TextBlock VerticalAlignment="Center" FontSize="12" Text="{Binding CityName}"/>
                            <TextBlock VerticalAlignment="Center" FontSize="12" Text="{Binding Area}"/>
                            <TextBlock VerticalAlignment="Center" FontSize="12" Text="{Binding Population}"/>                              
                            <TextBlock VerticalAlignment="Center" FontSize="12" Text="{Binding CityDetailsInfo.ClubsCount}"/>
                            <TextBlock VerticalAlignment="Center" FontSize="12" Text="{Binding CityDetailsInfo.PubsCount}"/>
                        </StackPanel>
                    </TabItem>
                </TabControl>
            </Grid>
        </Grid>
    </DockPanel>       
</DockPanel>