ADO.NET Entity Framework: Problem with updates on related objects - wpf

Question

I’m playing around with a little VB.NET app with WPf/MVVM and ADO.NET EF on a SQL Express-DB and I’m running into problems while trying to update related objects:
My DB has three tables “tb_Actors”, “tb_Movies” and a junction table “tb_movies_actors”. The EF designer creates two entities "Actors" und "Movies" and sets their navigation properties based on the foreign keys correctly. So was able to come up with a view which binds to a viewModels property which has all “Movies.Actors”.
The DataGrid in my view displays all actors correctly, and – if it’s a new actor which isn’t in my DB already- I am able to add new actors to a movie and persist the changes to the DB correctly.
However, if I want to add an actor to a movie who is already in my DB, I’ll get a double entry in my tb_actors table. First I’ve set the primary key fields (name and id) to UNIQUE, but then my code breaks. Then, I’ve added a little update routine which checks for each related actor of a movie if it’s a known actor, and changes the “new actors” id to the “old actors” id – this also breaks.
Is there a way to tell EF that it has to determine if an added related object (= already known actor added to movie) is already in the DB, and it therefore has to insert only a new entry to to junction table, but not to the related objects table?
My next step would be detaching the related objects and do all updates/inserts in my own data access code … but since I believe my problem is around a typical EF use case, there must be a more elegant way to deal with updates on related objects.
Any thoughts, answers, hints are highly appreciated!
* EDIT Here are the relevant code snippets *
1) I have the following LoadMovies Function in my MovieRepository data access class:
Private Function LoadMovies() As List(Of Movies)
movs = From m In dc.Movies.Include("Actors") Select m
Return movs.ToList
End Function
2) The following property of my viewModel exposes the actors related to a specific movie:
Public ReadOnly Property actors() As ICollectionView
Get
If evs Is Nothing Then
evs = New CollectionViewSource
evs.Source = _movie.Actors
End If
Return evs.View
End Get
End Property
3) In my MovieDetail view, I've a datagrid binding to the property:
<DataGrid Name="ActTestGrid" HorizontalAlignment="Left" VerticalAlignment="Stretch" ItemsSource="{Binding actors}" AutoGenerateColumns="False" Width="150" Height="120" Style="{StaticResource dgTemplate}" RowStyle="{StaticResource dgRowTemplate}" CellStyle="{StaticResource dgCellTemplate}" CanUserSortColumns="True" CanUserAddRows="True" CanUserDeleteRows="True" HeadersVisibility="None">
<DataGrid.Columns>
<DataGridTextColumn Header="Name" Binding="{Binding Path=name, UpdateSourceTrigger=PropertyChanged}" CanUserSort="true"/>
</DataGrid.Columns>
</DataGrid>
4) This is my updateMovie Function of my MovieRepository (as by now):
Public Sub UpdateMovie(ByVal movie As Movies)
If movie Is Nothing Then
Throw New ArgumentNullException("Movie")
Else
dc.SaveChanges()
End If
End Sub

Answer 1

EF does what it does and you can't tell it to do anymore, that is it won't validate your data. EF will insert what you tell it to insert (or try to). Data validation is your responsibility before you call save changes.
In order to get around this problem consider providing a combobox in the view with the list of actors names. The combobox has IsEditable set to true and the text property is bound to ActorName AS String in your ViewModel. If the user selects an existing actor EF will not try to insert a new one. If the user types in a new name EF will create a new actor.
Here is some code using a Brand Name:
Public Property BrandName() As String
Get
Return _brandName
End Get
Set
_brandName = value.Trim()
If _brandName <> String.Empty Then
Dim b As Brand = _brands.ToList().Find(Function(br) br.BrandName.ToUpper() = _brandName.ToUpper())
If b Is Nothing Then
Brand = New Brand()
Brand.BrandName = _brandName
Else
Brand = b
End If
Else
Brand = Nothing
End If
CheckIsDirty()
RaisePropertyChanged("BrandName")
End Set
End Property
And on the view:
<ComboBox Grid.Row="1" Grid.Column="0" Height="28" HorizontalAlignment="Stretch" Margin="110,0,28,0" VerticalAlignment="Top" TabIndex="1" ItemsSource="{Binding Brands}" DisplayMemberPath="BrandName" Text="{Binding BrandName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" IsEditable="True"/>