public function test_flag_dirty_attribute()
 {
     $author = new Author();
     $author->flag_dirty('some_inexistant_property');
     $this->assert_null($author->dirty_attributes());
     $this->assert_false($author->attribute_is_dirty('some_inexistant_property'));
 }
 public function test_flag_dirty()
 {
     $author = new Author();
     $author->flag_dirty('some_date');
     $this->assert_has_keys('some_date', $author->dirty_attributes());
 }