Create a "None" calculator for shipping and payment methods

Only for payment and shipping methods

app/models/calculator/none.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: false
+
+module Calculator
+  class None < Spree::Calculator
+    def self.description
+      I18n.t(:none)
+    end
+
+    def compute(_object = nil)
+      0
+    end
+  end
+end

config/application.rb

@@ -116,7 +116,8 @@ module Openfoodnetwork
           Calculator::FlexiRate,
           Calculator::PerItem,
           Calculator::PriceSack,
-          Calculator::Weight
+          Calculator::Weight,
+          Calculator::None
         ]
 
         app.config.spree.calculators.add_class('enterprise_fees')
@@ -135,7 +136,8 @@ module Openfoodnetwork
           Calculator::FlatRate,
           Calculator::FlexiRate,
           Calculator::PerItem,
-          Calculator::PriceSack
+          Calculator::PriceSack,
+          Calculator::None
         ]
 
         app.config.spree.calculators.add_class('tax_rates')

spec/system/admin/payment_method_spec.rb

@@ -251,6 +251,14 @@ describe '
     let!(:payment_method) { create(:payment_method, calculator: calculator) }
     before { login_as_admin_and_visit spree.edit_admin_payment_method_path payment_method }
 
+    it "handle the 'None' calculator" do
+      select2_select "None", from: 'calc_type'
+      click_button 'Update'
+      expect(page).to have_content("Payment Method has been successfully updated!")
+      expect(payment_method.reload.calculator_type).to eq "Calculator::None"
+      expect(page).to have_select "calc_type", selected: "None"
+    end
+
     context "using Flat Percent calculator" do
       before { select2_select "Flat Percent", from: 'calc_type' }
 

spec/system/admin/shipping_methods_spec.rb

@@ -83,6 +83,15 @@ describe 'shipping methods' do
 
       expect(@shipping_method.reload.calculator_type).to eq("Calculator::PerItem")
     end
+
+    it "handle when updating calculator type to 'None'" do
+      visit spree.edit_admin_shipping_method_path(@shipping_method)
+
+      select2_select 'None', from: 'calc_type'
+      click_button 'Update'
+      
+      expect(@shipping_method.reload.calculator_type).to eq "Calculator::None"
+    end
   end
 
   context "as an enterprise user", js: true do