Spree Calculator Exception Handling

calculator_decorator.rb

# app/models/spree/calculator_decorator.rb
Spree::Calculator.class_eval do
  class Spree::Calculator::CouldNotCalculateShipping < StandardError
  end
    
  class Spree::Calculator::CouldNotValidateOrder < StandardError
  end
end

checkout_controller_decorator.rb

# app/controllers/spree/checkout_controller_decorator.rb

Spree::CheckoutController.class_eval do
  
  rescue_from Spree::Calculator::CouldNotCalculateShipping do |exception|
    flash[:error] = 'Please verify your shipping address'
    redirect_to checkout_path
  end
    
  rescue_from Spree::Calculator::CouldNotValidateOrder do |exception|
    flash[:error] = 'Please verify your order'
    redirect_to cart_path
  end   
          
end   

table_rate.rb

# app/models/spree/calculator/table_rate.rb

module Spree
  class Calculator::TableRate < Calculator
    def self.description   
      "Table Rate"
    end                    

    def compute(object=nil)
      raise CouldNotValidateOrder, 'There was an error with your order.' unless object.present? and object.line_items.present?
      
      case object
        when Spree::Order
          # get the appropriate table based on the zone
          table = ShippingTable.find_by_zone_id(object.ship_address.zone.id)
          raise CouldNotCalculateShipping, 'No shipping table available.' if table == nil
          return table.compute_shipping_cost(object)
        when Spree::LineItem
          raise CouldNotValidateOrder, 'There was an error with your order.'
        when Spree::Shipment
          return 0.0
      end 
    end
  end
end