current_checkout.rb

module CurrentCheckout
  extend ActiveSupport::Concern

  included do
    before_action :set_current_checkout
  end

private

  def set_current_checkout
    @current_checkout = if session[:checkout] == 'buys'
      current_group_buy_cart
    elsif session[:checkout] == 'bros'
      current_bro_cart
    else
      current_cart
    end
  end
end

details_controller.rb

class Checkout::DetailsController < ApplicationController
  include CurrentCheckout
  before_action :set_shipping_address

  def show
  end

  def update
    if @shipping_address.update_attributes(shipping_address_params)
      redirect_to checkout_confirmation_url
    else
      render :show
    end
  end

private

  def shipping_address_params
    params.require(:shipping_address).permit(:full_name, :email, :street1, :street2, :city, :state, :zip, :phone, :residential, :country)
  end
  
  def set_shipping_address
    @shipping_address = @current_checkout.shipping_address
  end
end