WP_REST_Meta_Fields::update_multi_meta_value( int $object_id, string $meta_key, string $name, array $values )

	protected function update_multi_meta_value( $object_id, $meta_key, $name, $values ) {
		$meta_type = $this->get_meta_type();
		if ( ! current_user_can( "edit_{$meta_type}_meta", $object_id, $meta_key ) ) {
			return new WP_Error(
				'rest_cannot_update',
				/* translators: %s: custom field key */
				sprintf( __( 'Sorry, you are not allowed to edit the %s custom field.' ), $name ),
				array( 'key' => $name, 'status' => rest_authorization_required_code() )
			);
		}

		$current = get_metadata( $meta_type, $object_id, $meta_key, false );

		$to_remove = $current;
		$to_add    = $values;

		foreach ( $to_add as $add_key => $value ) {
			$remove_keys = array_keys( $to_remove, $value, true );

			if ( empty( $remove_keys ) ) {
				continue;
			}

			if ( count( $remove_keys ) > 1 ) {
				// To remove, we need to remove first, then add, so don't touch.
				continue;
			}

			$remove_key = $remove_keys[0];

			unset( $to_remove[ $remove_key ] );
			unset( $to_add[ $add_key ] );
		}

		// `delete_metadata` removes _all_ instances of the value, so only call once.
		$to_remove = array_unique( $to_remove );

		foreach ( $to_remove as $value ) {
			if ( ! delete_metadata( $meta_type, $object_id, wp_slash( $meta_key ), wp_slash( $value ) ) ) {
				return new WP_Error(
					'rest_meta_database_error',
					__( 'Could not update meta value in database.' ),
					array( 'key' => $name, 'status' => WP_Http::INTERNAL_SERVER_ERROR )
				);
			}
		}

		foreach ( $to_add as $value ) {
			if ( ! add_metadata( $meta_type, $object_id, wp_slash( $meta_key ), wp_slash( $value ) ) ) {
				return new WP_Error(
					'rest_meta_database_error',
					__( 'Could not update meta value in database.' ),
					array( 'key' => $name, 'status' => WP_Http::INTERNAL_SERVER_ERROR )
				);
			}
		}

		return true;
	}