Validating parent quantity domain

app/models/quantity.rb

@@ -12,4 +12,8 @@ class Quantity < ActiveRecord::Base
   validates :project, associated: true
   validates :name, presence: true, uniqueness: {scope: :project_id}
   validates :domain, inclusion: {in: domains.keys}
+  validates :parent, associated: true
+  validate if: -> { parent.present? } do
+    errors.add(:parent, :parent_domain_mismatch) unless domain == parent.domain
+  end
 end