上移下移功能

app/controllers/courses_controller.rb

@@ -288,7 +288,7 @@ class CoursesController < ApplicationController
     inform = @course.informs.find_by(id: params[:inform_id])
     next_inform = inform.next_inform
     ActiveRecord::Base.transaction do
-      if next_inform
+      if next_inform.blank?
         render_error('已经到达最顶部')
       else
         inform.update_attribute(:position, (position + 1))
@@ -302,7 +302,7 @@ class CoursesController < ApplicationController
     inform = @course.informs.find_by(id: params[:inform_id])
     last_inform = inform.last_inform
     ActiveRecord::Base.transaction do
-      if last_inform
+      if last_inform.blank?
         render_error('已经到达最底部')
       else
         inform.update_attribute(:position, (position - 1))