您似乎正在使用Rails 5教程，但是正在使用Rails4.在Rails 5中，所有模型都继承自ApplicationRecord，而Rails 4则继承自ApplicationRecord. c4>

It appears you're using the Rails 5 tutorial, but working with Rails 4. In Rails 5 all models inherit from ApplicationRecord, while Rails 4 from ActiveRecord::Base

立即修复:

class User < ActiveRecord::Base
...
end

长期修复，切换到Rails 5并学习Rails 5

Long term fix, switch to Rails 5 and learn with Rails 5