Populate combobox with collection other than the one the Grid it is in is bound to

I have the following code:

<Grid DataContext="{Binding ItemTypes}">
            ...

            <TextBlock Text="Name:" />
            <TextBox Grid.Column="2" Text="{Binding Name}" />

            <TextBlock Grid.Row="1" Text="Description:" />
            <TextBox Grid.Column="1" Grid.Row="1" Text="{Binding Description}" />

            <TextBlock Grid.Row="2" Text="Manufacturer:" />
            <ComboBox Grid.Column="1" Grid.Row="2" />

            <TextBlock Grid.Row="3" Text="Short Name:" />
            <TextBox Grid.Column="1" Grid.Row="3" Text="{Binding ShortName}" />
        </Grid>

The ItemTypes that the Grid's DataContext is set with, comes from a ViewModel that holds yet another collection. The Manufacturer Combobox that's inside this Grid needs to be filled with the other collection. I tried this:

     ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor,
 AncestorType=Window, AncestorLevel=1}, Path=DataContext.companies}"

But it didn't work. How can I get the combobox to fill with the other collection than the one the Grid is bound to?

ViewModel code:

    public class ItemTypeViewModel
    {
        #region private fields
        private ICollectionView collectionView;
        private IItemTypeService itemTypeService;
        #endregion

        #region automatic properties
        public ObservableCollection<ItemTypeViewModel> ItemTypes { get; private set; }
        public IEnumerable<Company> companies { get; private set; }
        #endregion properties

        #region constructors
        public ItemTypeAdminViewModel(IItemTypeService itemTypeService)
        {
            this.itemTypeService = itemTypeService;
            Initialize();
            collectionView = CollectionViewSource.GetDefaultView(ItemTypes);
        }
        #endregion

        #region private methods
        private void Initialize()
        {
            //TODO figure out if I should I wrap in Try/Catch here
            ItemTypes = new ObservableCollection<ItemTypeViewModel>(itemTypeService.GetItemTypes());
            companies = itemTypeService.GetCompanies();
        }
        #endregion

        #region commands
        public ICommand GoToFirst
        {
            get
            {
                return new RelayCommand(() => collectionView.MoveCurrentToFirst(),
                    () => collectionView.CurrentPosition >= 1);
            }
        }

        public ICommand GoToLast
        {
            get
            {
                return new RelayCommand(() => collectionView.MoveCurrentToLast(),
                    () => collectionView.CurrentPosition < (ItemTypes.Count - 1));
            }
        }

        public ICommand NextCommand
        {
            get
            {
                return new RelayCommand(() => 开发者_JS百科collectionView.MoveCurrentToNext(),
                    () => collectionView.CurrentPosition < (ItemTypes.Count - 1));
            }
        }

        public ICommand PreviousCommand
        {
            get
            {
                return new RelayCommand(() => collectionView.MoveCurrentToPrevious(),
                    () => collectionView.CurrentPosition >= 1);
            }
        }
        #endregion
    }
}

Assuming your view model is set to Window.DataContext and your other collection MyOtherCollection is a property in the view model. Change your ComboBox to the following:

<ComboBox Grid.Column="1" Grid.Row="2"
          ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=Window}, Path=DataContext.MyOtherCollection}" />

What's assigned to datacontext of the view in its constructor. If you assign view's datacontext to be the viewmodel through constructor injection, you should be able to directly bind as ItemsSource="{Binding companies}"

I would say just don't set the DataContext on the Grid. Instead, bind each item individually. It's difficult to give further advice unless you give further information - specifically, your VM.