title: Set ComboBox Selected Item/Text to Value path of ItemsSource
body: <p>I have an Entity table which lists ISO Country Codes and their descriptions in order to populate a ComboBox. I then have another table which sets a 'Nationality' to the selected value of the ComboBox. I have no foreign-key constraints between the IsoCountryCodes and my other table. </p>
  
  <p>I want to be able to set the SelectedItem of the ComboBox to the selected IsoCountryCode, but I want to save the SelectedValue to the 'Nationality' field. </p>
  
  <p>I've tried setting the SelectedItem of the ComboBox to my 'Nationality' (string) field but it is just doing a ToString() on the IsoCountryCode. When I set the 'Text' field instead of SelectedItem, it is saving the DisplayPath value to the 'Nationality' field.</p>
  
  <p>I was wondering if there was a way to get the ValuePath value to save to the field without any foreign key constraints, whilst still displaying a valid DisplayPath to the user?</p>
  
  <p>Example of IsoCountryCode table:</p>
  
  <pre><code>IsoCode     IsoDescription
  
  'GBR'      'United Kingdom'
  'USA'      'United States of America'
  </code></pre>
  
  <p>Example of the other table:</p>
  
  <pre><code>PassengerId  PassengerName  PassengerNationality   PassengerDocumentIssuingCountry
  1            'Carter, John'   'GBR'                'USA'
  </code></pre>
  
  <p>And my current Xaml:</p>
  
  <pre><code>&lt;xctk:WatermarkComboBox x:Name="uxNationalityCmbo" Margin="0,0,5,5" Watermark="Nationality" 
        Grid.Row="2" ItemsSource="{Binding CountryCodeCollection}" SelectedValuePath="IsoCode" DisplayMemberPath="IsoDescription" 
        IsTextSearchEnabled="True" Text="{Binding SelectedPassenger.PassengerNationality, TargetNullValue=''}"/&gt;
  </code></pre>
  
  <p>Changing the selected value now would to USA would change the entity PassengerNationality to 'United States of America', whereas I need 'USA'</p>