code: def getTransitionUsers(obj, action_id, last_user=False):
      
      workflow = getToolByName(obj, 'portal_workflow')
      users = []
      try:
          # https://jira.bikalabs.com/browse/LIMS-2242:
          # Sometimes the workflow history is inexplicably missing!
          review_history = list(workflow.getInfoFor(obj, 'review_history'))
      except WorkflowException:
          logger.error(
              "workflow history is inexplicably missing."
              " https://jira.bikalabs.com/browse/LIMS-2242")
          return users
      # invert the list, so we always see the most recent matching event
      review_history.reverse()
      for event in review_history:
          if event.get('action', '') == action_id:
              value = event.get('actor', '')
              users.append(value)
              if last_user:
                  return users
      return users
docs: This function returns a list with the users who have done the transition.
      :action_id: a sring as the transition id.
      :last_user: a boolean to return only the last user triggering the
          transition or all of them.
      :returns: a list of user ids.