Fixes #3931 - Changing the category of a Knowledge Base answer leads to alternative translation loop

app/controllers/knowledge_base/public/answers_controller.rb

@@ -13,7 +13,9 @@ class KnowledgeBase::Public::AnswersController < KnowledgeBase::Public::BaseCont
   private
 
   def render_alternative
-    @alternative = find_answer @knowledge_base.answers.eager_load(translations: :kb_locale), params[:answer], locale: false
+    answers = @knowledge_base.answers.where(category: params[:category]).eager_load(translations: :kb_locale)
+
+    @alternative = find_answer(answers, params[:answer], locale: false)
 
     raise ActiveRecord::RecordNotFound if !@alternative&.translations&.any?
 

spec/requests/knowledge_base_public/answers_spec.rb

@@ -35,4 +35,39 @@ RSpec.describe 'KnowledgeBase public answers', type: :request do
       end
     end
   end
+
+  describe '#render_alternative' do
+    context 'when a translation is available' do
+      before { create(:knowledge_base_translation, kb_locale: alternative_locale) }
+
+      it 'returns OK for published answer' do
+        get help_answer_path(alternative_locale.system_locale.locale, category, published_answer)
+        expect(response).to have_http_status :ok
+      end
+
+      it 'returns NOT FOUND for draft answer' do
+        get help_answer_path(alternative_locale.system_locale.locale, category, draft_answer)
+        expect(response).to have_http_status :not_found
+      end
+
+      # https://github.com/zammad/zammad/issues/3931
+      context 'when the category has been updated' do
+        let(:new_category) { create(:knowledge_base_category, knowledge_base: knowledge_base) }
+
+        it 'returns NOT FOUND for published answer if old category is used' do
+          published_answer.update! category_id: new_category.id
+
+          get help_answer_path(alternative_locale.system_locale.locale, category, published_answer)
+          expect(response).to have_http_status :not_found
+        end
+
+        it 'returns OK for published answer if new category is used' do
+          published_answer.update! category_id: new_category.id
+
+          get help_answer_path(alternative_locale.system_locale.locale, new_category, published_answer)
+          expect(response).to have_http_status :ok
+        end
+      end
+    end
+  end
 end