Clean `Topic#slug` when `SiteSetting.slug_generation_method` changes.

https://meta.discourse.org/t/removing-the-concept-of-slugs-for-some-languages/26643/24?u=tgxworld

app/models/topic.rb

@@ -33,6 +33,14 @@ class Topic < ActiveRecord::Base
 
   attr_accessor :allowed_user_ids, :tags_changed
 
+  DiscourseEvent.on(:site_setting_saved) do |site_setting|
+    if site_setting.name.to_s == "slug_generation_method" && site_setting.saved_change_to_value?
+      Scheduler::Defer.later("Null topic slug") do
+        Topic.update_all(slug: nil)
+      end
+    end
+  end
+
   def self.max_sort_order
     @max_sort_order ||= (2**31) - 1
   end

spec/models/topic_spec.rb

@@ -201,6 +201,26 @@ describe Topic do
         end
       end
     end
+
+    describe 'when updating SiteSetting.slug_generation_method' do
+      let(:title) { '熱帶風暴畫眉熱帶風暴畫眉熱帶風暴畫眉' }
+      let(:slug) { 'topic' }
+
+      it 'should clear the slug' do
+        topic.save!
+        expect(topic.title).to eq(title)
+        expect(topic.slug).to eq(slug)
+
+        begin
+          Scheduler::Defer.async = false
+          SiteSetting.update(name: 'slug_generation_method', value: 'encoded')
+          expect(topic.reload.read_attribute(:slug)).to eq(nil)
+        ensure
+          SiteSetting.where(name: 'slug_generation_method').destroy_all
+          Scheduler::Defer.async = true
+        end
+      end
+    end
   end
 
   context "updating a title to be shorter" do