feat: Add promotion code functionality to ticket orders

app/models/order_promotion_code.rb

@@ -0,0 +1,26 @@
+class OrderPromotionCode < ApplicationRecord
+  # Associations
+  belongs_to :order
+  belongs_to :promotion_code
+
+  # Validations
+  validates :order, presence: true
+  validates :promotion_code, presence: true
+
+  # Callbacks
+  after_create :apply_discount
+  after_create :increment_promotion_code_uses
+
+  private
+
+  def apply_discount
+    # Apply the discount to the order
+    discount_amount = promotion_code.discount_amount_cents
+    order.update!(total_amount_cents: [ order.total_amount_cents - discount_amount, 0 ].max)
+  end
+
+  def increment_promotion_code_uses
+    # Increment the uses count on the promotion code
+    promotion_code.increment!(:uses_count)
+  end
+end