title: How cancel SaveChanges
body: <p>I make first SaveChanges and flies exception (UpdateException).
  And I make second SaveChanges and againe fliyng first erorr.
  What to do about it</p>
  
  <pre><code>bool isUpdate = false;
  var resource = new Resource() { url = tbUrl.Text };
  //block1
  try
  {
      context.Resource.AddObject(resource);
      context.SaveChanges();
      isUpdate = true;
  }
  catch (UpdateException ex)
  {
  
  }
  
  //block2
  if (!isUpdate)
  {
      resource = (from res in context.Resource where res.url == tbUrl.Text select res).First();
      context.NameToResourcer.AddObject(new NameToResourcer()
                      {
                          id_resource = resource.id,
                          name = tag
                      });
      context.SaveChanges();//error!
  }
  </code></pre>