I like using service classes for things like this.

  class StoryService
    def self.assign_story(params)
      story = Story.find(params[:story_id])
      user = User.find(params[:user_id])
      story.assign_to(user)
    end
  end

This example is fairly minimal, but with service classes handling workflow and object coordination instead of the controller, the code is easier to test and reuse.