code: def migrate_doc(self, doc):
  		
  		orig_ver = doc.get(self.version_attribute_name, 0)
  		funcs = self._get_migrate_funcs(orig_ver, self.target_version)
  		for func in funcs:
  			func(self, doc)
  			doc[self.version_attribute_name] = func.target
  		return doc
docs: Migrate the doc from its current version to the target version
  		and return it.