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

app/models/quantity.rb

@@ -1,5 +1,6 @@
 class Quantity < ApplicationRecord
   ATTRIBUTES = [:name, :description, :parent_id]
+  attr_cached :depth, :pathname
 
   belongs_to :user, optional: true
   belongs_to :parent, optional: true, class_name: "Quantity"
@@ -70,10 +71,6 @@ class Quantity < ApplicationRecord
     self[:depth] = parent&.depth&.succ || 0
   end
 
-  def depth=(value)
-    raise ActiveRecord::ReadonlyAttributeError
-  end
-
   scope :defaults, ->{ where(user: nil) }
 
   # Return: ordered [sub]hierarchy