title: ModelState.IsValid is false because CreatedBy field is null
body: <p>I have an entity with, amongst others, the fields: <code>CreatedBy</code>, <code>CreatedAt</code>, <code>ChangedAt</code></p>
  
  <p>Now in the view, I do not want the user to fill these fields.
  My approach was to fill them on the HTTP POST Action, before <code>ModelState.IsValid</code> check is made and before saving the data to the database.</p>
  
  <p>However, <code>ModelState.IsValid</code> keeps returning false no matter what. What is the right way to implement this? Should I take the validation (<code>ModelState.IsValid</code>) from the POST action?</p>