Use ActiveModel::Model for validations on non-AR objects

booking_payments_controller.rb

class BookingPaymentsController < ApplicationController
  def create
    booking_payment = BookingPayment.new(booking_payment_params)
    booking_payment.pay

    respond_with payment, location: balance_trip_path
  end
  
  private
  
  def booking_payment_params
    params.require(:booking_payment).permit(
      :card_number,
      :card_expiry
    ).merge(
      ip_address: request.ip,
      booking: booking
    )
  end
  
  def booking
    # ...
  end
end

makes_booking_payment.rb

class BookingPayment
  include ActiveModel::Model
  
  attr_accessor :booking, :card_number, :card_expiry
  
  validates :booking, presence: true
  validates :card_number, presence: true, credit_card_number: true
  validates :card_expiry, presence: true
  
  def pay
    if valid? && payment_processed?
      create_transaction
      create_payment
    end
  end

  private
  
  def payment_processed?
    process_payment_response.success?
  end
  
  def process_payment_response
    @process_payment_response ||= process_payment.response
  end
  
  def process_payment
    PaymentProcessor.process(booking, card_number: card_number, card_expiry: card_expiry)
  end

  def create_transaction
    Transaction.record_purchase(booking, process_payment_response)
  end
  
  def create_payment
    # ...
  end
end