Source

File: wp-includes/rest-api/fields/class-wp-rest-meta-fields.php

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_values = get_metadata( $meta_type, $object_id, $meta_key, false );
		$subtype        = get_object_subtype( $meta_type, $object_id );

		$to_remove = $current_values;
		$to_add    = $values;

		foreach ( $to_add as $add_key => $value ) {
			$remove_keys = array_keys(
				array_filter(
					$current_values,
					function ( $stored_value ) use ( $meta_key, $subtype, $value ) {
						return $this->is_meta_value_same_as_stored_value( $meta_key, $subtype, $stored_value, $value );
					}
				)
			);

			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. Otherwise,
		 * `delete_metadata` will return false for subsequent calls of the same value.
		 * Use serialization to produce a predictable string that can be used by array_unique.
		 */
		$to_remove = array_map( 'maybe_unserialize', array_unique( array_map( 'maybe_serialize', $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',
					/* translators: %s: Custom field key. */
					sprintf( __( 'Could not update the meta value of %s in database.' ), $meta_key ),
					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',
					/* translators: %s: Custom field key. */
					sprintf( __( 'Could not update the meta value of %s in database.' ), $meta_key ),
					array(
						'key'    => $name,
						'status' => WP_Http::INTERNAL_SERVER_ERROR,
					)
				);
			}
		}

		return true;
	}