title: WPF SelectedItem not working in MVVM
body: <p>I'm trying to display the data from two sql ce 3.5 sp1 database tables linked with foreign key - Customers and Orders. When the customer is selected in a datadrig, I want the other grid to be populated with the Orders.
  I'm using a query:</p>
  
  <pre><code>var profiles = from c in db.Customers.Include("Orders")
                             select c;
  </code></pre>
  
  <p>And in my ViewModel:</p>
  
  <pre><code>private Models.Customers _selecteditem;
          public Models.Customers SelectedItem
          {
              get { return _selecteditem; }
  
          }
  </code></pre>
  
  <p>the view looks like this:</p>
  
  <pre><code>&lt;Grid&gt;
          &lt;toolkit:DataGrid  x:Name="dg1" ItemsSource="{Binding Customers}" SelectedItem="{Binding SelectedItem, mode=TwoWay}"&gt;
              &lt;/toolkit:DataGrid&gt;
          &lt;toolkit:DataGrid  x:Name="dg2" ItemsSource="{Binding Path=SelectedItem.Orders}"&gt;
          &lt;/toolkit:DataGrid&gt;
      &lt;/Grid&gt;
  </code></pre>
  
  <p>The error I'm getting is:</p>
  
  <pre><code>Warning 1   Field 'Clients.ViewModels.CustomerViewModel._selecteditem' is never assigned to, and will always have its default value null    
  </code></pre>
  
  <p>How to make it work correctly? When I just want to display Customers it is ok.
  Thanks for any suggestions.</p>