Extract out a DISPLAY_ON_OPTIONS constant in Spree::ShippingMethod instead of magic strings

app/models/spree/shipping_method.rb

@@ -3,7 +3,10 @@
 module Spree
   class ShippingMethod < ApplicationRecord
     include CalculatedAdjustments
-    DISPLAY = [:both, :front_end, :back_end].freeze
+    DISPLAY_ON_OPTIONS = {
+      both: "",
+      back_end: "back_end"
+    }.freeze
 
     acts_as_paranoid
     acts_as_taggable
@@ -27,6 +30,7 @@ module Spree
     validates :name, presence: true
     validate :distributor_validation
     validate :at_least_one_shipping_category
+    validates :display_on, inclusion: { in: DISPLAY_ON_OPTIONS.values }, allow_nil: true
 
     after_save :touch_distributors
 
@@ -101,12 +105,12 @@ module Spree
       ]
     end
 
-    def self.on_backend_query
-      "#{table_name}.display_on != 'front_end' OR #{table_name}.display_on IS NULL"
+    def self.backend
+      where("spree_shipping_methods.display_on = ?", DISPLAY_ON_OPTIONS[:back_end])
     end
 
-    def self.on_frontend_query
-      "#{table_name}.display_on != 'back_end' OR #{table_name}.display_on IS NULL"
+    def self.frontend
+      where("spree_shipping_methods.display_on IS NULL OR spree_shipping_methods.display_on = ''")
     end
 
     private

app/models/spree/shipping_rate.rb

@@ -8,7 +8,7 @@ module Spree
     scope :frontend,
           -> {
             includes(:shipping_method).
-              where(ShippingMethod.on_frontend_query).
+              merge(ShippingMethod.frontend).
               references(:shipping_method).
               order("cost ASC")
           }

app/views/spree/admin/shipping_methods/_form.html.haml

@@ -19,7 +19,7 @@
     .alpha.four.columns
       = f.label :display_on, t(:display)
     .omega.twelve.columns
-      = select(:shipping_method, :display_on, [[t(".both"), nil], [t(".back_end"), "back_end"]], {}, {class: 'select2 fullwidth'})
+      = select(:shipping_method, :display_on, Spree::ShippingMethod::DISPLAY_ON_OPTIONS.map { |key, value| [t(".#{key}"), value] }, {}, {class: 'select2 fullwidth'})
       = error_message_on :shipping_method, :display_on
 
   .row

spec/models/spree/shipping_method_spec.rb

@@ -143,6 +143,23 @@ module Spree
         expect(shipping_method.errors[:name].first).to eq "can't be blank"
       end
 
+      describe "#display_on" do
+        it "is valid when it's set to nil, an empty string or 'back_end'" do
+          shipping_method = build_stubbed(:shipping_method)
+          [nil, "", "back_end"].each do |display_on_option|
+            shipping_method.display_on = display_on_option
+            shipping_method.valid?
+            expect(shipping_method.errors[:display_on]).to be_empty
+          end
+        end
+
+        it "is not valid when it's set to an unknown value" do
+          shipping_method = build_stubbed(:shipping_method, display_on: "front_end")
+          expect(shipping_method).not_to be_valid
+          expect(shipping_method.errors[:display_on]).to eq ["is not included in the list"]
+        end
+      end
+
       context "shipping category" do
         it "validates presence of at least one" do
           shipping_method = build_stubbed(