ability to track resources used in conference

app/models/resource.rb

@@ -0,0 +1,10 @@
+class Resource < ActiveRecord::Base
+  belongs_to :conference
+  validate :used_less_than_quantity
+
+  private
+
+  def used_less_than_quantity
+    errors.add(:used, 'can not be higher than total quantity') unless used <= quantity
+  end
+end