Deprecate the creation of option_values without an option_type

It makes no sense to have dangling option_values without an associated
option_type.

Ref solidusio#3309 & solidusio#3517

api/app/controllers/spree/api/option_values_controller.rb

@@ -18,6 +18,13 @@ def show
       end
 
       def create
+        Spree::Deprecation.warn <<~MSG unless request.path.include?('option_types')
+          This route is deprecated, as it'll be no longer possible to create an
+          option_value without an associated option_type. Please, use instead:
+
+            POST api/option_types/{option_type_id}/option_values
+        MSG
+
         authorize! :create, Spree::OptionValue
         @option_value = scope.new(option_value_params)
         if @option_value.save

api/config/routes.rb

@@ -54,6 +54,7 @@
     resources :option_types do
       resources :option_values
     end
+    # TODO: Remove :create once option_type is required on Solidus v4.0
     resources :option_values
 
     get '/orders/mine', to: 'orders#mine', as: 'my_orders'

api/openapi/solidus-api.oas.yml

@@ -2544,6 +2544,8 @@ paths:
           $ref: '#/components/responses/unprocessable-entity'
       summary: Create option value
       description: |-
+        **DEPRECATED**: Use *POST /option_types/{option_type_id}/option_values* instead
+
         Creates an option value.
 
         Only users with the `create` permission on `Spree::OptionValue` can perform this action.

api/spec/requests/spree/api/option_values_spec.rb

@@ -110,7 +110,9 @@ module Spree::Api
           expect(option_value.name).to eq("Option Value")
         end
 
-        it "can create an option value" do
+        it "can create but deprecates creating an option value without option type" do
+          expect(Spree::Deprecation).to receive(:warn).with(/deprecated/).at_least(:once)
+
           post spree.api_option_values_path, params: { option_value: {
                                 name: "Option Value",
                                 presentation: 'option value'

core/app/models/spree/option_value.rb

@@ -2,6 +2,8 @@
 
 module Spree
   class OptionValue < Spree::Base
+    # TODO: Remove optional on Solidus v4.0. Don't forget adding a migration to
+    # enforce at the database layer
     belongs_to :option_type, class_name: 'Spree::OptionType', inverse_of: :option_values, optional: true
     acts_as_list scope: :option_type
 
@@ -13,7 +15,14 @@ class OptionValue < Spree::Base
 
     after_save :touch, if: :saved_changes?
     after_touch :touch_all_variants
+    after_save do
+      Spree::Deprecation.warn <<~MSG if option_type.nil?
+        Having an option_value with no associated option_type will be deprecated
+        on Solidus v4.0
+      MSG
+    end
 
+    # TODO: Remove allow_nil once option_type is required on Solidus v4.0
     delegate :name, :presentation, to: :option_type, prefix: :option_type, allow_nil: true
 
     self.whitelisted_ransackable_attributes = %w[name presentation]

core/spec/models/spree/option_value_spec.rb

@@ -49,4 +49,10 @@
       expect(subject).to eq "Size - S"
     end
   end
+
+  it 'deprecates creating an option_value with no associated option_type' do
+    expect(Spree::Deprecation).to receive(:warn).with(/deprecated/)
+
+    create(:option_value, option_type: nil)
+  end
 end