Inherit models from ApplicationRecord instead of ActiveRecord::Base.

app/models/favorite.rb

@@ -1,4 +1,4 @@
-class Favorite < ActiveRecord::Base
+class Favorite < ApplicationRecord
   belongs_to :post
   belongs_to :user
   scope :for_user, lambda {|user_id| where("user_id % 100 = #{user_id.to_i % 100} and user_id = #{user_id.to_i}")}