Fixes #11379: Made the subtopics method

app/controllers/tag_controller.rb

@@ -568,4 +568,13 @@ def fetch_counts
   end
 
   def topic_tree; end
+
+  def subtopics
+    # here, we look for this special tag marking a node as having a parent
+    # collect the "slugs" i.e. the unique part of the URL, which should correspond to a tagname
+    subtopic_names = Tag.find_by(name: "parent:#{params[:topic]}")
+      .nodes
+      .collect(&:slug)
+    @subtopics = Tag.find_by(name: subtopic_names)
+    render "tags/subtopics", layout: false
 end

config/routes.rb

@@ -383,6 +383,7 @@
   post 'comment/react/update/:id' => 'comment#react_update'
 
   get 'topic-tree' => 'tag#topic_tree'
+  get 'tags/subtopic/:topic' => 'tag#subtopics'
 
   # Sample resource route (maps HTTP verbs to controller actions automatically):
   #   resources :products

test/functional/tag_controller_test.rb

@@ -458,6 +458,15 @@ def setup
     assert_not_nil assigns(:notes)
     assert (notes & expected).present?
   end
+
+  test 'subtopics partial' do
+    get :subtopics,
+        params: {
+          topic: 'test'
+        }
+    assert :success
+    assert_not_nil assigns(:subtopics)
+  end
 
   test 'shows suggested tags' do
     get :suggested, params: { id: 'spectr' }