protected function get_activity_user_id( $activity_id ) {
		// If there is an existing activity/user ID mapping, just return the user ID.
		if ( ! empty( $this->activity_user_id[$activity_id] ) ) {
			return $this->activity_user_id[$activity_id];

		/*
		 * We don't have a mapping. This means the $activity_id is not on the current
		 * page of results, so fetch its details from the database.
		 */
		} else {
			$activity = bp_activity_get_specific( array( 'activity_ids' => $activity_id, 'show_hidden' => true, 'spam' => 'all', ) );

			/*
			 * If, somehow, the referenced activity has been deleted, leaving its associated
			 * activities as orphans, use the logged in user's ID to avoid errors.
			 */
			if ( empty( $activity['activities'] ) )
				return bp_loggedin_user_id();

			// Store the new activity/user ID mapping for any later re-use.
			$this->activity_user_id[ $activity['activities'][0]->id ] = $activity['activities'][0]->user_id;

			// Return the user ID.
			return $activity['activities'][0]->user_id;
		}
	}