payment_notification.rb

class PaymentNotification < ActiveRecord::Base
  belongs_to :cart
  belongs_to :product
  serialize :params
  after_create :mark_cart_as_purchased, :update_qty
 # after_create :send_mail
 


  private

  	  	
  def mark_cart_as_purchased
  	if status == "Completed"
  	  cart.update_attribute(:purchased_at, Time.current)
  	end
  end

  def send_mail
  	if status == "Completed" && cart.purchased_at
  		OrderNotifier.received(@order).deliver_now
        OrderNotifier.notify(@order).deliver_now
    end
  end

  def update_qty
  	if status == "Completed"
  		line_item = LineItem.find_by(id: line_item_id)
  		line_item.upd(params[:quantity])
  		line_item.update_attribute(:status, "purchased")
  	end
  end

end

payment_notifications_controller.rb

class PaymentNotificationsController < ApplicationController
  protect_from_forgery except: [:create]
  before_action :extract_ipn_items_params, only: [:create]
  
  ITEM_PARAM_PREFIXES = ["item_name", "item_number", "quantity", "option_name"]


   def create
     extracted_values = extract_ipn_items_params
     # Now use extracted_values instead of params in the stuff below
    PaymentNotification.create!(params: params, cart_id: params[:invoice], status: params[:payment_status], transaction_id: params[:txn_id], 
      #line_item_id: params[:item_number], product_title: params[:item_name], option_name: params[:option_name], quantity: params[:quantity], 
      first_name: params[:first_name], last_name: params[:last_name], email: params[:payer_email], address_name: params[:address_name], 
      address_street: params[:address_street], address_city: params[:address_city], address_state: params[:address_state], 
      address_zip: params[:address_zip], address_country: params[:address_country])
    render nothing: true
  end

  def details
    params.extract_ipn_items_params
    PaymentNotification.update_attributes(line_item_id: params[:item_number], product_title: params[:item_name], option_name: params[:option_name], quantity: params[:quantity])
  end

  private

    def extract_ipn_items_params
      mod_params = Hash.new{|k, v| k[v] = {} }

      ITEM_PARAM_PREFIXES.each do |item_data_key|
          key_tracker = 1
          loop do
              current_key = (item_data_key + key_tracker.to_s).to_sym
              if params.include? current_key
                  mod_params[key_tracker][item_data_key] = params[current_key]
              else
                  break
              end
              key_tracker += 1
          end
      end
      mod_params
    end
  
end