Destroy topic's collection when topic is destroyed

plugin.rb

@@ -225,7 +225,10 @@
     actors.as_json
   end
 
-  Topic.has_one :activity_pub_object, class_name: "DiscourseActivityPubCollection", as: :model
+  Topic.has_one :activity_pub_object,
+                class_name: "DiscourseActivityPubCollection",
+                as: :model,
+                dependent: :destroy
   Topic.include DiscourseActivityPub::AP::ModelHelpers
 
   add_to_class(:topic, :activity_pub_taxonomies) do

spec/models/topic_spec.rb

@@ -41,6 +41,13 @@
     end
   end
 
+  describe "#destroy!" do
+    it "destroys associated collections" do
+      topic1.destroy!
+      expect(DiscourseActivityPubCollection.exists?(collection1.id)).to eq(false)
+    end
+  end
+
   describe "move_posts" do
     before { toggle_activity_pub(category1, publication_type: "full_topic") }