title: Only Edit some fields and "SaveChanges" fill the missing fields with default values
body: <p>I have a model class,</p>
  
  <pre><code>public class MyModel 
  { 
      public int A { get; set; }
      public int B { get; set; }
      public int C { get; set; }
      public DateTime D { get; set; }
  }
  </code></pre>
  
  <p>In the edit screen, I only need to edit `A and B. And the following will overwrite C and D with default values (0 and '0001-01-01'. How to keep the existed values (in the database table) for C and D besides include them in the view and hide them?</p>
  
  <pre><code>[HttpPost]
  public ActionResult Edit(MyModel myModel)
  {
      if (ModelState.IsValid)
      {
          _db_Entry(myModel).State = EntityState.Modified;
          _db.SaveChanges();
      }
      return View(myModel);
  }
  </code></pre>