It looks like it's there to use a custom setter that coerces to lowercase:
```
     @property
     def code(self):
         return self.code_

     @code.setter
      def code(self, val):
         self.code_ = val.lower() if val else None
```
Is the issue here that the actual resulting document doesn't save `@property`s and confusingly refers to `code_`?