Problem binding DataGridComboBoxColumn.ItemsSource

I have 3 tables: Item - which is the DataContext - it has a navigation column Group Group - has a navigation column Category.

I want to have in the DataGrid both (Category & Group) columns and when I choose a category it should display in the group col only the Category.Groups.

Here is the code I am working on:

<tk:DataGrid AutoGenerateColumns="False" ItemsSource="{Binding}">
    <tk:DataGrid.Columns>

        <!--Works-->
        <tk:DataGridComboBoxColumn                                        
            Header="Categroy" 
            DisplayMemberPath="Title"                    
            SelectedValuePath="CategoryId"
            SelectedValueBinding="{Binding Group.Category.CategoryId}"
            ItemsSource="{Binding Context.Categories, 
                Source={x:Static Application.Current}}"
        />


        <!--Look at these two things:-->

        <!--This does work-->
        <tk:DataGridTemplateColumn>
            <tk:DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <ItemsControl
                        ItemsSource="{Binding Group.Category.Groups}">
                        <ItemsControl.ItemTemplate>
                            <DataTemplate DataType="{x:Type data:Group}">
                                <TextBlock Text="{Binding Title}"/>
                            </DataTemplate>
                        </ItemsControl.ItemTemplate>
                    </ItemsControl>
                </DataTemplate>
            </tk:DataGridTemplateColumn.CellTemplate>
        </tk:DataGridTemplateColumn>

        <!--But this does NOT work, even it's the same source-->
        <!--Notice I even tried a dummy converter and doesnt reach there-->
        <tk:DataGridComboBoxColumn 
            Header="Group" 
            DisplayMemberPath="Title"
            SelectedValuePath="GroupId"
            ItemsSource="{Binding Group.Category.Groups,
                Converter={StaticResource DummyConverter}}"
            SelectedValueBinding="{Binding Group.GroupId}"
            />

    </tk:DataGrid.Columns>
</tk:DataGrid>

Update
Would you say the problem is that the ItemsSource property cannot be set to a non-static Binding? I suspect so because even I set the ItemsSource to {Binding} with the DummyConverter it doesn't stop in the converter; and in the Category ComboBox it works fine.

The columns in the datagrid don't have a datacontext, as they are never added to the visual tree. sound a bit weird but have a look at vince's blog, its got a good example of the visual layout. once the grid is drawn the cells have a data context and you can set the combo boxes items source in them using normal bindings (not static resources..)

You can access the combo box items source as such:

<dg:DataGridComboBoxColumn>
   <dg:DataGridComboBoxColumn.EditingElementStyle>
      <Style TargetType="ComboBox">
         <Setter Property="ItemsSource" Value="{Binding Path=MyBindingPath}" />
      </Style>
   </dg:DataGridComboBoxColumn.EditingElementStyle>
</dg:DataGridComboBoxColumn>

Have a look here and also here for some code. You will also need to set the items source for the non edting element as in this post

I was using MVVM and I wanted to bind the ItemSource of the column to a collection of objects in the window data context. I must have tried 10 different ways and nothing worked until I found this answer.

The trick is to define a CollectionViewSource outside the grid and then reference it inside the grid using StaticResource. For example,

<Window.Resources>
    <CollectionViewSource x:Key="ItemsCVS" Source="{Binding MyItems}" />
</Window.Resources>
<!-- ... -->
<DataGrid ItemsSource="{Binding MyRecords}">
    <DataGridComboBoxColumn Header="Column With Predefined Values"
                            ItemsSource="{Binding Source={StaticResource ItemsCVS}}"
                            SelectedValueBinding="{Binding MyItemId}"
                            SelectedValuePath="Id"
                            DisplayMemberPath="StatusCode" />
</DataGrid>