Problem binding DataGridComboBoxColumn.ItemsSource - wpf

Question

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.

Answer 1

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

Answer 2

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>