var result = await SignInManager.PasswordSignInAsync(model.Username,
                                                     model.Password,
                                                     model.RememberMe,
                                                     shouldLockout: false);

// Determine if user has entered their UserName or Email address.
// TODO: research if there is a more efficient way to do this.
using (var db = new ApplicationDbContext())
{
  model.Username = (db.Users.Any(p => p.UserName == model.Username)) ?
  model.Username :
  (db.Users.Any(p => p.Email == model.Username)) ?
  db.Users.SingleOrDefault(p => p.Email == model.Username).UserName :
  model.Username;
}