Cached attribute definition (attr_cached)

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