当Ruby的model名字出错时,在现实view时显示错误的提示
app/controllers/courses_controller.rb:1:in `<top (required)>'
app/controllers/courses_controller.rb:1:in `<top (required)>'
undefined method `key?' for nil:NilClass
app/controllers/courses_controller.rb:1:in `<top (required)>'
app/controllers/courses_controller.rb:1:in `<top (required)>'
undefined method `key?' for nil:NilClass