code: def update(self, document_id, update_spec, namespace, timestamp):
          
          document = self.doc_dict[document_id].doc
          updated = self.apply_update(document, update_spec)
          if "_id" in updated:
              updated.pop("_id")
          updated[self.unique_key] = document_id
          self.upsert(updated, namespace, timestamp)
          return updated
docs: Apply updates given in update_spec to the document whose id
          matches that of doc.