title: InnerException in Entity Framework when I try add an entity
body: <p>I'm trying to insert a entity into my database with Entity Framework. See below:</p>
  
  <pre><code>private void InsertarIntoDataBase()
  {
  
      Juego game = db.games.First(g =&gt; g.Id == anId);
      Day d = new Day {
                         DayNumber = this.Number,
                         Game = game // This is a relationship. One Day has one game 
                      };
      db.AddToDay(d);
      db.SaveChanges();
  }
  </code></pre>
  
  <p>This is always for an insert, not an update. The first time that I run my app, it works, afterwards it stops working, throwing this exception <code>An error occurred while updating the entries. See the InnerException for details.</code>. (I'm really sure that I did not change anything). </p>
  
  <p>Why does the framework think I'm updating? And what am I wrong?</p>