Fix serialization of added_to_collection notifications (#38612)

app/models/notification.rb

@@ -147,6 +147,15 @@ class Notification < ApplicationRecord
     end
   end
 
+  def target_collection
+    case type
+    when :added_to_collection
+      collection_item&.collection
+    when :collection_update
+      collection
+    end
+  end
+
   class << self
     def browserable(types: [], exclude_types: [], from_account_id: nil, include_filtered: false)
       requested_types = if types.empty?

app/models/notification_group.rb

@@ -48,6 +48,7 @@ class NotificationGroup < ActiveModelSerializers::Model
 
   delegate :type,
            :target_status,
+           :target_collection,
            :report,
            :account_relationship_severance_event,
            :account_warning,

app/serializers/rest/notification_group_serializer.rb

@@ -14,7 +14,7 @@ class REST::NotificationGroupSerializer < ActiveModel::Serializer
   belongs_to :account_relationship_severance_event, key: :event, if: :relationship_severance_event?, serializer: REST::AccountRelationshipSeveranceEventSerializer
   belongs_to :account_warning, key: :moderation_warning, if: :moderation_warning_event?, serializer: REST::AccountWarningSerializer
   belongs_to :generated_annual_report, key: :annual_report, if: :annual_report_event?, serializer: REST::AnnualReportEventSerializer
-  belongs_to :collection, if: :collection_type?, serializer: REST::CollectionSerializer
+  belongs_to :target_collection, key: :collection, if: :collection_type?, serializer: REST::CollectionSerializer
 
   def sample_account_ids
     object.sample_accounts.pluck(:id).map(&:to_s)

app/serializers/rest/notification_serializer.rb

@@ -11,7 +11,7 @@ class REST::NotificationSerializer < ActiveModel::Serializer
   belongs_to :report, if: :report_type?, serializer: REST::ReportSerializer
   belongs_to :account_relationship_severance_event, key: :event, if: :relationship_severance_event?, serializer: REST::AccountRelationshipSeveranceEventSerializer
   belongs_to :account_warning, key: :moderation_warning, if: :moderation_warning_event?, serializer: REST::AccountWarningSerializer
-  belongs_to :collection, if: :collection_type?, serializer: REST::CollectionSerializer
+  belongs_to :target_collection, key: :collection, if: :collection_type?, serializer: REST::CollectionSerializer
 
   def id
     object.id.to_s