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

config/locales/en.yml

@@ -5,6 +5,13 @@ en:
   field_action: 'Action'
   field_domain: 'Domain'
   field_parent_quantity: 'Parent'
+  activerecord:
+    errors:
+      models:
+        quantity:
+          attributes:
+            parent:
+              parent_domain_mismatch: 'parent quantity has to be in the same domain'
   body_trackers:
     index:
       heading: 'Summary'