Federate Add when item is added to Collection (#37823)

app/serializers/activitypub/add_featured_item_serializer.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class ActivityPub::AddFeaturedItemSerializer < ActivityPub::Serializer
+  include RoutingHelper
+
+  attributes :type, :actor, :target
+  has_one :object, serializer: ActivityPub::FeaturedItemSerializer
+
+  def type
+    'Add'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(collection.account)
+  end
+
+  def target
+    ActivityPub::TagManager.instance.uri_for(collection)
+  end
+
+  private
+
+  def collection
+    @collection ||= object.collection
+  end
+end

app/serializers/activitypub/featured_collection_serializer.rb

@@ -1,22 +1,6 @@
 # frozen_string_literal: true
 
 class ActivityPub::FeaturedCollectionSerializer < ActivityPub::Serializer
-  class FeaturedItemSerializer < ActivityPub::Serializer
-    attributes :type, :featured_object, :featured_object_type
-
-    def type
-      'FeaturedItem'
-    end
-
-    def featured_object
-      ActivityPub::TagManager.instance.uri_for(object.account)
-    end
-
-    def featured_object_type
-      object.account.actor_type || 'Person'
-    end
-  end
-
   attributes :id, :type, :total_items, :name, :attributed_to,
              :sensitive, :discoverable, :published, :updated
 
@@ -25,7 +9,7 @@ class ActivityPub::FeaturedCollectionSerializer < ActivityPub::Serializer
 
   has_one :tag, key: :topic, serializer: ActivityPub::NoteSerializer::TagSerializer
 
-  has_many :collection_items, key: :ordered_items, serializer: FeaturedItemSerializer
+  has_many :collection_items, key: :ordered_items, serializer: ActivityPub::FeaturedItemSerializer
 
   def id
     ActivityPub::TagManager.instance.uri_for(object)

app/serializers/activitypub/featured_item_serializer.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ActivityPub::FeaturedItemSerializer < ActivityPub::Serializer
+  attributes :type, :featured_object, :featured_object_type
+
+  def type
+    'FeaturedItem'
+  end
+
+  def featured_object
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+
+  def featured_object_type
+    object.account.actor_type || 'Person'
+  end
+end

app/services/add_account_to_collection_service.rb

@@ -9,7 +9,11 @@ class AddAccountToCollectionService
 
     raise Mastodon::NotPermittedError, I18n.t('accounts.errors.cannot_be_added_to_collections') unless AccountPolicy.new(@collection.account, @account).feature?
 
-    create_collection_item
+    @collection_item = create_collection_item
+
+    distribute_add_activity if @account.local? && Mastodon::Feature.collections_federation_enabled?
+
+    @collection_item
   end
 
   private
@@ -20,4 +24,12 @@ class AddAccountToCollectionService
       state: :accepted
     )
   end
+
+  def distribute_add_activity
+    ActivityPub::AccountRawDistributionWorker.perform_async(activity_json, @collection.account_id)
+  end
+
+  def activity_json
+    ActiveModelSerializers::SerializableResource.new(@collection_item, serializer: ActivityPub::AddFeaturedItemSerializer, adapter: ActivityPub::Adapter).to_json
+  end
 end

spec/serializers/activitypub/add_featured_item_serializer_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::AddFeaturedItemSerializer do
+  subject { serialized_record_json(object, described_class, adapter: ActivityPub::Adapter) }
+
+  let(:tag_manager) { ActivityPub::TagManager.instance }
+  let(:collection) { Fabricate(:collection) }
+  let(:object) { Fabricate(:collection_item, collection:) }
+
+  it 'serializes to the expected json' do
+    expect(subject).to include({
+      'type' => 'Add',
+      'actor' => tag_manager.uri_for(collection.account),
+      'target' => tag_manager.uri_for(collection),
+      'object' => a_hash_including({
+        'type' => 'FeaturedItem',
+      }),
+    })
+
+    expect(subject).to_not have_key('id')
+    expect(subject).to_not have_key('published')
+    expect(subject).to_not have_key('to')
+    expect(subject).to_not have_key('cc')
+  end
+end

spec/services/add_account_to_collection_service_spec.rb

@@ -20,6 +20,12 @@ RSpec.describe AddAccountToCollectionService do
         expect(new_item.state).to eq 'accepted'
         expect(new_item.account).to eq account
       end
+
+      it 'federates an `Add` activity', feature: :collections_federation do
+        subject.call(collection, account)
+
+        expect(ActivityPub::AccountRawDistributionWorker).to have_enqueued_sidekiq_job
+      end
     end
 
     context 'when given an account that is not featureable' do