FIX: permalinks should work for Discourse categories

app/controllers/list_controller.rb

@@ -228,11 +228,11 @@ class ListController < ApplicationController
     parent_category_id = nil
     if parent_slug_or_id.present?
       parent_category_id = Category.query_parent_category(parent_slug_or_id)
-      raise Discourse::NotFound if parent_category_id.blank?
+      redirect_or_not_found and return if parent_category_id.blank?
     end
 
     @category = Category.query_category(slug_or_id, parent_category_id)
-    raise Discourse::NotFound if !@category
+    redirect_or_not_found and return if !@category
 
     @description_meta = @category.description_text
     guardian.ensure_can_see!(@category)
@@ -308,4 +308,23 @@ class ListController < ApplicationController
     periods
   end
 
+  def redirect_or_not_found
+    url = request.fullpath
+    permalink = Permalink.find_by_url(url)
+
+    if permalink.present?
+      # permalink present, redirect to that URL
+      if permalink.external_url
+        redirect_to permalink.external_url, status: :moved_permanently
+      elsif permalink.target_url
+        redirect_to "#{Discourse::base_uri}#{permalink.target_url}", status: :moved_permanently
+      else
+        raise Discourse::NotFound
+      end
+    else
+      # redirect to 404
+      raise Discourse::NotFound
+    end
+  end
+
 end