DataGridComboBoxColumn binding issue - wpf

Question

I have DataGridComboBoxColumn in DataGrid:
<DataGridComboBoxColumn Header="{x:Static Properties:Resources.Unit}" Width="Auto"
SelectedValueBinding="{Binding UnitUnitId}"
SelectedValuePath="UnitId"
DisplayMemberPath="Name" >
<DataGridComboBoxColumn.ElementStyle>
<Style TargetType="ComboBox">
<Setter Property="ItemsSource" Value="{Binding Path=DataContext.Units,RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/>
</Style>
</DataGridComboBoxColumn.ElementStyle>
<DataGridComboBoxColumn.EditingElementStyle>
<Style TargetType="ComboBox">
<Setter Property="ItemsSource" Value="{Binding Path=DataContext.Units,RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/>
</Style>
</DataGridComboBoxColumn.EditingElementStyle>
</DataGridComboBoxColumn>
and ComboBox:
<ComboBox x:Name="unitBox" ItemsSource="{Binding Path=Units}"
SelectedItem="{Binding TaxSubGroup.Unit}"
Grid.Row="2"
Grid.Column="1" Margin="11,11,0,0" HorizontalAlignment="Stretch">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Converter={x:Static c:UnitToStringConverter.Default}}"/>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
Units property:
public ObservableCollection<Unit> Units
{
get
{
return _units;
}
set
{
if(_units!=value)
{
_units = value;
RaisePropertyChanged("Units");
}
}
}
when i'm changing ComboBoxes value, DataGridComboBoxColumn's value updating automatically, but changing DataGrid's comboboxcolumn's value not updating ComboBoxes value. Why?
UPDATED
The problem was in DataGridComboBoxCulumn's SelectedValueBinding property. All i needed is to add UpdateSourceTrigger property:
SelectedValueBinding="{Binding UnitUnitId,UpdateSourceTrigger=PropertyChanged}"

Answer 1

Are you updating TaxSubGroup.Unit in the setter of UnitUnitId property?
In your DataContext model class do something similar to this...
public int UnitUnitId
{
get
{
return this.unitID;
}
set
{
this.unitID = value;
this.TaxSubGroup.Unit = Units.FirstOrDefault(u => u.UnitID == value);
this.NotifyPropertyChange("UnitUnitId");
}
}
This will match SelectedItem of Combobox by reference.