how to save a relation?

Hello,
I've the following tables

Dashboard

CREATE TABLE [dbo].[DBOARD](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [DESCRIZIONE] [varchar](50) NOT NULL,
 CONSTRAINT [PK_DBS] PRIMARY KEY CLUSTERED
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]

Layout

CREATE TABLE [dbo].[LAYOUT](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [DESCRIZIONE] [varchar](50) NULL,
 CONSTRAINT [PK_LAYOUT] PRIMARY KEY CLUSTERED
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]


Dashboard_Layout

CREATE TABLE [dbo].[LAYOUT_DBOARD](
    [ID_DBOARD] [int] NOT NULL,
    [ID_LAYOUT] [int] NOT NULL,
 CONSTRAINT [PK_LAYOUTDBOARD] PRIMARY KEY CLUSTERED
(
    [ID_DBOARD] ASC,
    [ID_LAYOUT] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[LAYOUT_DBOARD]  WITH CHECK ADD  CONSTRAINT [FK_DBOARD_LAYOUT] FOREIGN KEY([ID_LAYOUT])
REFERENCES [dbo].[LAYOUT] ([ID])
GO

ALTER TABLE [dbo].[LAYOUT_DBOARD] CHECK CONSTRAINT [FK_DBOARD_LAYOUT]
GO

ALTER TABLE [dbo].[LAYOUT_DBOARD]  WITH CHECK ADD  CONSTRAINT [FK_LAYOUT_DBOARD] FOREIGN KEY([ID_DBOARD])
REFERENCES [dbo].[DBOARD] ([ID])
GO

ALTER TABLE [dbo].[LAYOUT_DBOARD] CHECK CONSTRAINT [FK_LAYOUT_DBOARD]
GO

I've a silverlight application that permits me to choose which layout is applied to a dashboard...

I my XAML application for getting the selected Item I do

   <telerik:RadComboBox Grid.Row="1" Grid.Column="1" SelectedItem="{Binding LAYOUT[0]}" DisplayMemberPath="DESCRIZIONE"
                                             ItemsSource="{Binding DataContext.ListaLayout,Mode=OneWay,RelativeSource={RelativeSource AncestorType=UserControl}}"
                                             Style="{StaticResource RadComboBoxMetroStyle}" Width="300" HorizontalAlignment="Left" ></telerik:RadComboBox>

This loads in the dropdown the current selected item, when I choose another one and try to save I have still the old selected one...
since for saving I'm doing

  public OperationResult CreateDashBoard(DBOARD dashboard, Action<IEnumerable> onSuccess = null, Action<Exception> onFail = null)
        {
            entityManagerProvider.Manager.AddEntity(dashboard);

            return AlwaysCompletedOperationResult.Instance;
        }

the dashboard.LAYOUT[0] is still the old value I've set... how do I manage MxN relations when changing the selected Item?

Thanks