title: Add migration not recognized my changes when I add class to my model but ok when I add property to an existing class
body: <p>I try to add a class and apply add-migration to update database but the migration script generated is up and down empty.</p>
  
  <p>I have deleted migration history table and update database, then apply modifications to my model by adding new class and generate add-migration script but it's KO.</p>
  
  <p>Really strange is when I add a property to an existing class, add-migration detect the changes and the script generated is not empty.</p>
  
  <p>please help</p>