feat(promotion code): Promotion code system done

I added the features for users to use promotion code
and for promoters to create on their events.
May be rewrite to discount code?

app/models/promotion_code.rb

@@ -12,9 +12,28 @@ class PromotionCode < ApplicationRecord
   before_create :increment_uses_count
 
   # Associations
+  belongs_to :user
+  belongs_to :event
   has_many :order_promotion_codes
   has_many :orders, through: :order_promotion_codes
 
+  # Instance methods
+  def discount_amount_euros
+    discount_amount_cents / 100.0
+  end
+
+  def active?
+    active && (expires_at.nil? || expires_at > Time.current)
+  end
+
+  def expired?
+    expires_at.present? && expires_at < Time.current
+  end
+
+  def can_be_used?
+    active? && (usage_limit.nil? || uses_count < usage_limit)
+  end
+
   private
 
   def increment_uses_count