Add experimental server-side notification grouping (#29889)

app/models/notification.rb

@@ -13,6 +13,7 @@
 #  from_account_id :bigint(8)        not null
 #  type            :string
 #  filtered        :boolean          default(FALSE), not null
+#  group_key       :string
 #
 
 class Notification < ApplicationRecord
@@ -136,6 +137,67 @@ class Notification < ApplicationRecord
       end
     end
 
+    # This returns notifications from the request page, but with at most one notification per group.
+    # Notifications that have no `group_key` each count as a separate group.
+    def paginate_groups_by_max_id(limit, max_id: nil, since_id: nil)
+      query = reorder(id: :desc)
+      query = query.where(id: ...max_id) if max_id.present?
+      query = query.where(id: (since_id + 1)...) if since_id.present?
+
+      unscoped
+        .with_recursive(
+          grouped_notifications: [
+            query
+              .select('notifications.*', "ARRAY[COALESCE(notifications.group_key, 'ungrouped-' || notifications.id)] groups")
+              .limit(1),
+            query
+              .joins('CROSS JOIN grouped_notifications')
+              .where('notifications.id < grouped_notifications.id')
+              .where.not("COALESCE(notifications.group_key, 'ungrouped-' || notifications.id) = ANY(grouped_notifications.groups)")
+              .select('notifications.*', "array_append(grouped_notifications.groups, COALESCE(notifications.group_key, 'ungrouped-' || notifications.id))")
+              .limit(1),
+          ]
+        )
+        .from('grouped_notifications AS notifications')
+        .order(id: :desc)
+        .limit(limit)
+    end
+
+    # Differs from :paginate_groups_by_max_id in that it gives the results immediately following min_id,
+    # whereas since_id gives the items with largest id, but with since_id as a cutoff.
+    # Results will be in ascending order by id.
+    def paginate_groups_by_min_id(limit, max_id: nil, min_id: nil)
+      query = reorder(id: :asc)
+      query = query.where(id: (min_id + 1)...) if min_id.present?
+      query = query.where(id: ...max_id) if max_id.present?
+
+      unscoped
+        .with_recursive(
+          grouped_notifications: [
+            query
+              .select('notifications.*', "ARRAY[COALESCE(notifications.group_key, 'ungrouped-' || notifications.id)] groups")
+              .limit(1),
+            query
+              .joins('CROSS JOIN grouped_notifications')
+              .where('notifications.id > grouped_notifications.id')
+              .where.not("COALESCE(notifications.group_key, 'ungrouped-' || notifications.id) = ANY(grouped_notifications.groups)")
+              .select('notifications.*', "array_append(grouped_notifications.groups, COALESCE(notifications.group_key, 'ungrouped-' || notifications.id))")
+              .limit(1),
+          ]
+        )
+        .from('grouped_notifications AS notifications')
+        .order(id: :asc)
+        .limit(limit)
+    end
+
+    def to_a_grouped_paginated_by_id(limit, options = {})
+      if options[:min_id].present?
+        paginate_groups_by_min_id(limit, min_id: options[:min_id], max_id: options[:max_id]).reverse
+      else
+        paginate_groups_by_max_id(limit, max_id: options[:max_id], since_id: options[:since_id]).to_a
+      end
+    end
+
     def preload_cache_collection_target_statuses(notifications, &_block)
       notifications.group_by(&:type).each do |type, grouped_notifications|
         associations = TARGET_STATUS_INCLUDES_BY_TYPE[type]

app/models/notification_group.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class NotificationGroup < ActiveModelSerializers::Model
+  attributes :group_key, :sample_accounts, :notifications_count, :notification
+
+  def self.from_notification(notification)
+    if notification.group_key.present?
+      # TODO: caching and preloading
+      sample_accounts = notification.account.notifications.where(group_key: notification.group_key).order(id: :desc).limit(3).map(&:from_account)
+      notifications_count = notification.account.notifications.where(group_key: notification.group_key).count
+    else
+      sample_accounts = [notification.from_account]
+      notifications_count = 1
+    end
+
+    NotificationGroup.new(
+      notification: notification,
+      group_key: notification.group_key || "ungrouped-#{notification.id}",
+      sample_accounts: sample_accounts,
+      notifications_count: notifications_count
+    )
+  end
+
+  delegate :type,
+           :target_status,
+           :report,
+           :account_relationship_severance_event,
+           to: :notification, prefix: false
+end