Clean up notifications if a user can't see a topic

app/controllers/topics_controller.rb

@@ -61,6 +61,14 @@ class TopicsController < ApplicationController
     perform_show_response
 
     canonical_url absolute_without_cdn(@topic_view.canonical_path)
+  rescue Discourse::InvalidAccess => ex
+
+    if current_user
+      # If the user can't see the topic, clean up notifications for it.
+      Notification.remove_for(current_user.id, params[:topic_id])
+    end
+
+    raise ex
   end
 
   def wordpress

app/models/notification.rb

@@ -65,6 +65,12 @@ class Notification < ActiveRecord::Base
     result
   end
 
+  # Clean up any notifications the user can no longer see. For example, if a topic was previously
+  # public then turns private.
+  def self.remove_for(user_id, topic_id)
+    Notification.where(user_id: user_id, topic_id: topic_id).delete_all
+  end
+
   # Be wary of calling this frequently. O(n) JSON parsing can suck.
   def data_hash
     @data_hash ||= begin