Cached attribute definition (attr_cached)

app/models/application_record.rb

@@ -1,3 +1,17 @@
 class ApplicationRecord < ActiveRecord::Base
+  class << self
+    # Cached attribute has non-user assignable value calculated from other
+    # attributes' values on create/update. This simplifies and speeds up
+    # actions, especially for recursively calculated values. Because value can
+    # be changed on update, it is not same as #attr_readonly.
+    def attr_cached(*names)
+      names.each { |name| alias_method :"#{name}=", :assign_cached_attribute }
+    end
+  end
+
+  def assign_cached_attribute(value)
+    raise ActiveRecord::ReadonlyAttributeError
+  end
+
   primary_abstract_class
 end