How to add selection changed event for combobox inside the datagrid

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MVVM

i am working on mvvm application, in which i have bind the datagrid. in the datagrid bind the combobox, which is bind with different datasource, i want to add selection changed event for that combobox. and how to find the selectedvalue of that combobox with row id of the data grid.
i have tried below code:
xaml code:

<DataGridTemplateColumn MinWidth="100" Header="Grade Scenario" >
                            <DataGridTemplateColumn.CellTemplate>
                                <DataTemplate>
                                    <ComboBox DisplayMemberPath="GradeScenarioID"   SelectedValuePath="GradeScenarioID" ItemsSource="{Binding Path=DataContext.GradeScenarioData, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserControl}}}" >
                                    </ComboBox>
                                </DataTemplate>
                            </DataGridTemplateColumn.CellTemplate>
                        </DataGridTemplateColumn>

What I have tried:

viewmodel code:

 public ObservableCollection<assetsubclass> AssetSubClassData
       {
           get
           {
               return _assetSubClassData;
           }
           set
           {
               _assetSubClassData = value;
               RaisePropertyChanged("AssetSubClassData");
           }
       }


private async void AssetClassSelectionChanged(RoutedEventArgs e)
       {
           try
           {
               Messenger.Default.Send<bool>(true, BroadcastMessageToken.ShowBusyIndicator);
               if (!string.IsNullOrWhiteSpace(ClassID))
               {
                   var assetSubClasses = await _dataService.GetCollectionByParentId<assetsubclass>(ClassID);
                   if (assetSubClasses != null)
                   {
                       AssetSubClassData = new ObservableCollection<assetsubclass>(assetSubClasses.ToList());
                   }
               }
               var gradeScenario = await _dataService.GetCollection<data.sql.gradescenario>();
               if (gradeScenario != null)
               {
                   GradeScenarioData = new ObservableCollection<data.sql.gradescenario>(gradeScenario.ToList());
               }
           }
           catch (Exception ex)
           {
               ExceptionDetails(ex);
           }
           finally
           {
               Messenger.Default.Send<bool>(false, BroadcastMessageToken.ShowBusyIndicator);
           }
       }