Question

I’m trying to get a DataGridComboBoxColumn working with my ViewModel. Everything appears to work correctly but when I change the value of the combo box, the entity isn’t changed.

The datacontext of the window has these properties:

ItemsSource

Public Property AllEnergySources() As ObservableCollection(Of EnergySourceViewModel)

SelectedItemBinding

  Private _CurrentEnergySource As EnergySourceViewModel
    Public Property CurrentEnergySource() As EnergySourceViewModel
        Get
            Return _CurrentEnergySource
        End Get
        Set(ByVal value As EnergySourceViewModel)
            _CurrentEnergySource = value
            OnPropertyChanged("CurrentEnergySource")
        End Set
    End Property

I feel the problem lies with how I populate CurrentEnergySource in the ViewModel that is the DataContext:

Sub New(SelectedEntity as EquipmentEnergySource)
     AllEnergySources = New ObservableCollection(Of EnergySourceViewModel)

    //Select all EnergySources from the EntityFramework
     Dim EnergyEntities = From esr in db.EnergySources Select esr

                //Loop through to convert Entity POCO to Collection of ViewModels
                For Each es In EnergyEntities
                    _AllEnergySources.Add(New EnergySourceViewModel(es))

                    //Optionally Set the newly created ViewModel to SelectedItemBinding object
                    If es.EnergySourceID = SelectedEntity.EnergySourceID Then
                        _CurrentEnergySource = _AllEnergySources.Last
                    End If
                Next
End Sub

When I create the backing collection for the combobox, if the model is the selected one, I set that viewmodel to be the CurrentEnergySource but after that point it is disconnected(and that’s the problem)

What should I reference in CurrentEnergySource so that it updates the model when the combo box changes?

Answer 1

My answer is you need to change the foreign key manually (I now change it in the setter of CurrentEnergySource, which is the SelectedItemBinding bound property)

     Private _CurrentEnergySource As EnergySourceViewModel
    Public Property CurrentEnergySource() As EnergySourceViewModel
        Get
            Return _CurrentEnergySource
        End Get
        Set(ByVal value As EnergySourceViewModel)
            _CurrentEnergySource = value
            Me.Model.EnergySourceID = value.Model.EnergySourceID
            OnPropertyChanged("CurrentEnergySource")
        End Set
    End Property

On load, populate the private backing store _CurrentEnergySource instead of the property to avoid all objects starting out with modified state