docs: def save(self, db):
          """
          Saves the object to given database. Usage::
  
              item = Item(title=u'Hello')
              item.save(db)
  
          Collection name is taken from :attr:`MongoBoundDictMixin.collection`.
          """
          assert self.collection
  
          self._ensure_indexes(db)
  
          # XXX self.structure belongs to StructuredDictMixin !!
          outgoing = dict(dict_to_db(self, self.structure))
  
          object_id = db[self.collection].save(outgoing)
  
          if self.get('_id') is None:
              self['_id'] = object_id
          else:
              pass
  
          return object_id