DEV: Implement wizard replacement validation in Topic Creation

config/locales/server.en.yml

@@ -55,6 +55,8 @@ en:
       liquid_syntax_error: "Liquid syntax error in %{attribute}: %{message}"
       subscription: "%{type} %{property} usage is not supported on your subscription"
       not_permitted_for_guests: "%{object_id} is not permitted when guests can access the wizard"
+    error_messages:
+      wizard_replacing_composer: "A wizard is set to replace the composer in this category. You cannot create a topic here."
 
   site_settings:
     custom_wizard_enabled: "Enable custom wizards."

lib/custom_wizard/extensions/topic_extension.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module CustomWizardTopicExtension
+  extend ActiveSupport::Concern
+
+  included { before_validation :check_wizard_replacement, on: :create }
+
+  def check_wizard_replacement
+    if wizard_replacing_composer?(self.category_id)
+      self.errors.add(
+        :base,
+        I18n.t('wizard.error_messages.wizard_replacing_composer')
+      )
+    end
+  end
+
+  def wizard_replacing_composer?(category_id)
+    return false unless category_id
+
+    category = Category.find(category_id)
+    category.custom_fields['create_topic_wizard'].present?
+  end
+end

plugin.rb

@@ -91,6 +91,7 @@ after_initialize do
     ../lib/custom_wizard/extensions/invites_controller.rb
     ../lib/custom_wizard/extensions/users_controller.rb
     ../lib/custom_wizard/extensions/guardian.rb
+    ../lib/custom_wizard/extensions/topic_extension.rb
     ../lib/custom_wizard/extensions/custom_field/preloader.rb
     ../lib/custom_wizard/extensions/custom_field/serializer.rb
     ../lib/custom_wizard/extensions/custom_field/extension.rb
@@ -200,6 +201,7 @@ after_initialize do
   ::InvitesController.prepend InvitesControllerCustomWizard
   ::UsersController.prepend CustomWizardUsersController
   ::Guardian.prepend CustomWizardGuardian
+  ::Topic.include CustomWizardTopicExtension
 
   full_path = "#{Rails.root}/plugins/discourse-custom-wizard/assets/stylesheets/wizard/wizard_custom.scss"
   if Stylesheet::Importer.respond_to?(:plugin_assets)