code: def update(self, *fields):
          
          from mongoframes.queries import to_refs
  
          assert '_id' in self._document, "Can't update documents without `_id`"
  
          # Send update signal
          signal('update').send(self.__class__, frames=[self])
  
          # Check for selective updates
          if len(fields) > 0:
              document = {}
              for field in fields:
                  document[field] = self._path_to_value(field, self._document)
          else:
              document = self._document
  
          # Prepare the document to be updated
          document = to_refs(document)
          document.pop('_id', None)
  
          # Update the document
          self.get_collection().update_one({'_id': self._id}, {'$set': document})
  
          # Send updated signal
          signal('updated').send(self.__class__, frames=[self])
docs: Update this document. Optionally a specific list of fields to update can
          be specified.