SubmitChanges does not update foreign key to the database - vb.net

Question

My problem occurs in a Windows Phone 8.1 Silverlight VB application. The CatDataContext defines a table Books with items Title and a foreign key _seriesID, with belongs to a table Series.
<Table()>
Public Class Series
Implements INotifyPropertyChanged, INotifyPropertyChanging
' Define ID: private field, public property, and database column.
Private _seriesID As Integer
<Column(IsPrimaryKey:=True, IsDbGenerated:=True, DbType:="INT NOT NULL Identity", CanBeNull:=False,
AutoSync:=AutoSync.OnInsert)>
Public Property SeriesID() As Integer
Get
Return _seriesID
End Get
Set(ByVal value As Integer)
If _seriesID <> value Then
NotifyPropertyChanging("SeriesID")
_seriesID = value
NotifyPropertyChanged("SeriesID")
End If
End Set
End Property
' Define name: private field, public property, and database column.
Private _Name As String
<Column()>
Public Property Name() As String
Get
Return _Name
End Get
Set(ByVal value As String)
If _Name <> value Then
NotifyPropertyChanging("Name")
_Name = value
NotifyPropertyChanged("Name")
End If
End Set
End Property
#Region "INotifyPropertyChanged Members"
Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
' Used to notify that a property changed
Private Sub NotifyPropertyChanged(ByVal propertyName As String)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
End Sub
#End Region
#Region "INotifyPropertyChanging Members"
Public Event PropertyChanging As PropertyChangingEventHandler Implements INotifyPropertyChanging.PropertyChanging
' Used to notify that a property is about to change
Private Sub NotifyPropertyChanging(ByVal propertyName As String)
RaiseEvent PropertyChanging(Me, New PropertyChangingEventArgs(propertyName))
End Sub
#End Region
End Class
<Table()>
Public Class Book
Implements INotifyPropertyChanged, INotifyPropertyChanging
' Define ID: private field, public property, and database column.
Private _bookID As Integer
<Column(IsPrimaryKey:=True, IsDbGenerated:=True, DbType:="INT NOT NULL Identity", CanBeNull:=False,
AutoSync:=AutoSync.OnInsert)>
Public Property BookID() As Integer
Get
Return _bookID
End Get
Set(ByVal value As Integer)
If _bookID <> value Then
NotifyPropertyChanging("BookID")
_bookID = value
NotifyPropertyChanged("BookID")
End If
End Set
End Property
' Define title: private field, public property, and database column.
Private _title As String
<Column()>
Public Property Title() As String
Get
Return _title
End Get
Set(ByVal value As String)
If _title <> value Then
NotifyPropertyChanging("Title")
_title = value
NotifyPropertyChanged("Title")
End If
End Set
End Property
' Internal column for the associated series ID value.
<Column()>
Friend _seriesID As Integer
Private _series As EntityRef(Of Series)
<Association(Storage:="_series", ThisKey:="_seriesID", OtherKey:="SeriesID")>
Public Property BookSeries() As Series
Get
Return _series.Entity
End Get
Set(ByVal value As Series)
NotifyPropertyChanging("BookSeries")
_series.Entity = value
If value IsNot Nothing Then
_seriesID = value.SeriesID
End If
NotifyPropertyChanged("BookSeries")
End Set
End Property
#Region "INotifyPropertyChanged Members"
Public Event PropertyChanged As PropertyChangedEventHandler Implements NotifyPropertyChanged.PropertyChanged
' Used to notify that a property changed
Private Sub NotifyPropertyChanged(ByVal propertyName As String)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
End Sub
#End Region
#Region "INotifyPropertyChanging Members"
Public Event PropertyChanging As PropertyChangingEventHandler Implements INotifyPropertyChanging.PropertyChanging
' Used to notify that a property is about to change
Private Sub NotifyPropertyChanging(ByVal propertyName As String)
RaiseEvent PropertyChanging(Me, New PropertyChangingEventArgs(propertyName))
End Sub
#End Region
Updating of the field Title alone, or the fields Title and _seriesID works fine. However when I only change the _seriesID then no update of the underlying database is done. In this case .GetModifiedMembers shows no modifications.
A reference to a demo project showing this problem is given here: demo project.
Thanks for your attention.

Answer 1

_seriesID (in Book) is just a member variable. You can set it from outside Book because it is Friend, but further nothing happens.
Title, on the other hand, is a property that fires NotifyPropertyChanged. That means that the context is notified that the Book object has been modified if you change Title.
So if you change _seriesID and Title, the Book object is marked as modified and saved, along with the changed value of _seriesID. But if you change _seriesID alone, the object remains 'unchanged'.
I think this is generated code (LINQ-to-SQL? I don't really recognize it) and you shouldn't modify it manually. If you want _seriesID to be changed, you have to set BookSeries.