code: def update_many(cls, documents, *fields):
          
          from mongoframes.queries import to_refs
  
          # Ensure all documents have been converted to frames
          frames = cls._ensure_frames(documents)
  
          all_count = len(documents)
          assert len([f for f in frames if '_id' in f._document]) == all_count, \
                  "Can't update documents without `_id`s"
  
          # Send update signal
          signal('update').send(cls, frames=frames)
  
          # Prepare the documents to be updated
  
          # Check for selective updates
          if len(fields) > 0:
              documents = []
              for frame in frames:
                  document = {'_id': frame._id}
                  for field in fields:
                      document[field] = cls._path_to_value(
                          field,
                          frame._document
                          )
                  documents.append(to_refs(document))
          else:
              documents = [to_refs(f._document) for f in frames]
  
          # Update the documents
          for document in documents:
              _id = document.pop('_id')
              cls.get_collection().update(
                  {'_id': _id}, {'$set': document})
  
          # Send updated signal
          signal('updated').send(cls, frames=frames)
docs: Update multiple documents. Optionally a specific list of fields to
          update can be specified.