Add experimental server-side notification grouping (#29889)

app/serializers/rest/notification_group_serializer.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+class REST::NotificationGroupSerializer < ActiveModel::Serializer
+  attributes :group_key, :notifications_count, :type
+
+  attribute :page_min_id, if: :paginated?
+  attribute :page_max_id, if: :paginated?
+  attribute :latest_page_notification_at, if: :paginated?
+
+  has_many :sample_accounts, serializer: REST::AccountSerializer
+  belongs_to :target_status, key: :status, if: :status_type?, serializer: REST::StatusSerializer
+  belongs_to :report, if: :report_type?, serializer: REST::ReportSerializer
+  belongs_to :account_relationship_severance_event, key: :event, if: :relationship_severance_event?, serializer: REST::AccountRelationshipSeveranceEventSerializer
+
+  def status_type?
+    [:favourite, :reblog, :status, :mention, :poll, :update].include?(object.type)
+  end
+
+  def report_type?
+    object.type == :'admin.report'
+  end
+
+  def relationship_severance_event?
+    object.type == :severed_relationships
+  end
+
+  def page_min_id
+    range = instance_options[:group_metadata][object.group_key]
+    range.present? ? range[:min_id].to_s : object.notification.id.to_s
+  end
+
+  def page_max_id
+    range = instance_options[:group_metadata][object.group_key]
+    range.present? ? range[:max_id].to_s : object.notification.id.to_s
+  end
+
+  def latest_page_notification_at
+    range = instance_options[:group_metadata][object.group_key]
+    range.present? ? range[:latest_notification_at] : object.notification.created_at
+  end
+
+  def paginated?
+    instance_options[:group_metadata].present?
+  end
+end