Destroy dependent notifications of a collection (#39429)

app/models/collection.rb

@@ -33,6 +33,7 @@ class Collection < ApplicationRecord
   has_many :accepted_collection_items, -> { accepted }, class_name: 'CollectionItem', inverse_of: :collection # rubocop:disable Rails/HasManyOrHasOneDependent
   has_many :collection_reports, dependent: :delete_all
   has_many :accounts, -> { merge(CollectionItem.pending_or_accepted) }, through: :collection_items
+  has_many :notifications, as: :activity, dependent: :destroy
 
   validates :name, presence: true
   validates :name, length: { maximum: 40 }, if: :local?

app/models/notification.rb

@@ -136,7 +136,7 @@ class Notification < ApplicationRecord
     belongs_to :generated_annual_report, inverse_of: false
     belongs_to :quote, inverse_of: :notification
     belongs_to :collection_item, inverse_of: false # TODO: have an inverse?
-    belongs_to :collection, inverse_of: false # TODO: have an inverse?
+    belongs_to :collection, inverse_of: :notifications
   end
 
   validates :type, inclusion: { in: TYPES }

spec/models/collection_spec.rb

@@ -221,4 +221,19 @@ RSpec.describe Collection do
       expect(subject.to_log_permalink).to eq ActivityPub::TagManager.instance.uri_for(subject)
     end
   end
+
+  describe '#destroy' do
+    let(:collection) { Fabricate(:collection) }
+
+    before do
+      Fabricate(:notification, activity: collection, type: :added_to_collection)
+      Fabricate(:notification, activity: collection, type: :collection_update)
+    end
+
+    it 'removes the collection and all notifications that reference it' do
+      expect { collection.destroy }
+        .to change(described_class, :count).by(-1)
+        .and change(Notification, :count).by(-2)
+    end
+  end
 end