public function get_attribute_keys() {
		if ( isset( self::$memo[ get_called_class() ][ __METHOD__ ] ) ) {
			return self::$memo[ get_called_class() ][ __METHOD__ ];
		}

		return self::$memo[ get_called_class() ][ __METHOD__ ]
			= array_merge(
				$this->fillable,
				$this->guarded,
				$this->get_compute_methods()
			);
	}