def subscription_index(options = {})
      target_index = subscriptions.filtered_by_options(options)
      target_index = options[:reverse] ? target_index.earliest_order : target_index.latest_order
      target_index = target_index.with_target if options[:with_target]
      options[:limit].present? ? target_index.limit(options[:limit]).to_a : target_index.to_a
    end
def subscription_index(options = {})
      target_index = subscriptions.filtered_by_options(options)
      target_index = options[:reverse] ? target_index.earliest_order : target_index.latest_order
      target_index = target_index.with_target if options[:with_target]
      options[:limit].present? ? target_index.limit(options[:limit]).to_a : target_index.to_a
    end
Gets configured subscription index of the target.

 @example Get configured subscription index of the @user
   @subscriptions = @user.subscription_index

 @param [Hash] options Options for subscription index
 @option options [Integer]    :limit                  (nil)   Limit to query for subscriptions
 @option options [Boolean]    :reverse                (false) If subscription index will be ordered as earliest first
 @option options [String]     :filtered_by_key        (nil)   Key of the notification for filter
 @option options [Array|Hash] :custom_filter          (nil)   Custom subscription filter (e.g. ["created_at >= ?", time.hour.ago])
 @option options [Boolean]    :with_target            (false) If it includes target with subscriptions
 @return [Array<Notificaion>] Configured subscription index of the target