Respect empty multiselect creation meta (#122)

includes/Fields/FieldDefaults.php

@@ -171,13 +171,15 @@ public static function normalize( mixed $raw, string $type, ?array $option_value
 	/**
 	 * Applies all supported defaults for a collection to one newly-created row.
 	 *
-	 * @param int $collection_id Collection post id.
-	 * @param int $row_id        Row post id.
+	 * @param int      $collection_id      Collection post id.
+	 * @param int      $row_id             Row post id.
+	 * @param string[] $explicit_meta_keys Field meta keys explicitly provided during creation.
 	 */
-	public static function apply_to_row( int $collection_id, int $row_id ): void {
-		$field_ids = array_map( 'intval', get_post_meta( $collection_id, 'fields', false ) );
-		$store     = new FieldValueStore();
-		$status    = (string) get_post_status( $row_id );
+	public static function apply_to_row( int $collection_id, int $row_id, array $explicit_meta_keys = array() ): void {
+		$field_ids          = array_map( 'intval', get_post_meta( $collection_id, 'fields', false ) );
+		$explicit_meta_keys = array_fill_keys( array_map( 'strval', $explicit_meta_keys ), true );
+		$store              = new FieldValueStore();
+		$status             = (string) get_post_status( $row_id );
 
 		foreach ( $field_ids as $field_id ) {
 			$type = (string) get_post_meta( $field_id, 'type', true );
@@ -186,6 +188,10 @@ public static function apply_to_row( int $collection_id, int $row_id ): void {
 			}
 
 			$key = Relations::meta_key( $field_id );
+			if ( isset( $explicit_meta_keys[ $key ] ) ) {
+				continue;
+			}
+
 			if ( count( get_metadata( 'post', $row_id, $key, false ) ) > 0 ) {
 				continue;
 			}

includes/PostType/CollectionEntries.php

@@ -381,6 +381,30 @@ private function collection_id_for_entry_post_type( string $post_type ): int {
 		return 0;
 	}
 
+	/**
+	 * Returns a set of field meta keys that use multi-row post meta storage.
+	 *
+	 * @param string $post_type Entry post type.
+	 * @return array<string,bool>
+	 */
+	private function multi_value_meta_keys_for_entry_post_type( string $post_type ): array {
+		$collection_id = $this->collection_id_for_entry_post_type( $post_type );
+		if ( $collection_id < 1 ) {
+			return array();
+		}
+
+		$keys = array();
+		foreach ( get_post_meta( $collection_id, 'fields', false ) as $field_id ) {
+			$field_id = (int) $field_id;
+			$type     = (string) get_post_meta( $field_id, 'type', true );
+			if ( 'multiselect' === $type || ( 'relation' === $type && Relations::relation_is_multiple( $field_id ) ) ) {
+				$keys[ Relations::meta_key( $field_id ) ] = true;
+			}
+		}
+
+		return $keys;
+	}
+
 	/**
 	 * Records the current user as the last editor of an entry.
 	 *
@@ -508,11 +532,42 @@ public function register_for_collection( WP_Post $collection ): void {
 		}
 
 		if ( empty( self::$default_hooks[ $post_type ] ) ) {
+			add_filter( "rest_pre_insert_{$post_type}", array( $this, 'prepare_meta_for_defaults_before_rest_insert' ), 10, 2 );
 			add_action( "rest_after_insert_{$post_type}", array( $this, 'apply_defaults_after_rest_insert' ), 10, 3 );
 			self::$default_hooks[ $post_type ] = true;
 		}
 	}
 
+	/**
+	 * Tracks explicit creation meta and removes empty multi-value fields before core REST deletes them.
+	 *
+	 * @param object          $prepared_post Prepared post object.
+	 * @param WP_REST_Request $request       REST request.
+	 */
+	public function prepare_meta_for_defaults_before_rest_insert( object $prepared_post, WP_REST_Request $request ): object {
+		if ( ! empty( $prepared_post->ID ) || ! empty( $prepared_post->id ) ) {
+			return $prepared_post;
+		}
+
+		$meta = $request->get_param( 'meta' );
+		if ( ! is_array( $meta ) ) {
+			return $prepared_post;
+		}
+
+		$explicit_meta_keys = array_keys( $meta );
+		$multi_meta_keys    = $this->multi_value_meta_keys_for_entry_post_type( (string) $prepared_post->post_type );
+		foreach ( $meta as $key => $value ) {
+			if ( array() === $value && isset( $multi_meta_keys[ (string) $key ] ) ) {
+				unset( $meta[ $key ] );
+			}
+		}
+
+		$request->set_param( 'meta', $meta );
+		$request->set_param( '_cortext_explicit_meta_keys', $explicit_meta_keys );
+
+		return $prepared_post;
+	}
+
 	public function apply_defaults_after_rest_insert( WP_Post $post, WP_REST_Request $request, bool $creating ): void {
 		if ( ! $creating ) {
 			return;
@@ -523,7 +578,13 @@ public function apply_defaults_after_rest_insert( WP_Post $post, WP_REST_Request
 			return;
 		}
 
-		FieldDefaults::apply_to_row( $collection_id, (int) $post->ID );
+		$explicit_meta_keys = $request->get_param( '_cortext_explicit_meta_keys' );
+		if ( ! is_array( $explicit_meta_keys ) ) {
+			$meta               = $request->get_param( 'meta' );
+			$explicit_meta_keys = is_array( $meta ) ? array_keys( $meta ) : array();
+		}
+
+		FieldDefaults::apply_to_row( $collection_id, (int) $post->ID, $explicit_meta_keys );
 	}
 
 	public function assign_menu_order_on_insert( int $post_id, WP_Post $post, bool $update ): void {

tests/php/test-rest-rows-controller.php

@@ -188,6 +188,47 @@ public function test_explicit_creation_meta_wins_over_field_default(): void {
 		$this->assertSame( 'Provided', get_post_meta( $row_id, "field-{$field_id}", true ) );
 	}
 
+	public function test_explicit_empty_multiselect_meta_wins_over_field_default(): void {
+		wp_set_current_user( $this->create_user( 'author' ) );
+		$collection_id = $this->create_collection_with_slug( 'Empty Multi Defaults', 'emptymultidefs' );
+		$field_id      = $this->create_collection_field(
+			$collection_id,
+			'Tags',
+			'multiselect',
+			array(
+				'options' => wp_json_encode(
+					array(
+						array(
+							'value' => 'a',
+							'label' => 'A',
+						),
+					)
+				),
+			)
+		);
+		update_post_meta( $field_id, 'default_value', '{"mode":"value","value":["a"]}' );
+
+		$GLOBALS['wp_rest_server'] = new WP_REST_Server();
+		( new RowsController() )->register();
+		do_action( 'rest_api_init' );
+
+		$request = new WP_REST_Request( 'POST', '/wp/v2/crtxt_emptymultidefs' );
+		$request->set_body_params(
+			array(
+				'status' => 'private',
+				'title'  => 'Empty tags row',
+				'meta'   => array(
+					"field-{$field_id}" => array(),
+				),
+			)
+		);
+		$response = rest_do_request( $request );
+		$this->assertSame( 201, $response->get_status() );
+
+		$row_id = (int) $response->get_data()['id'];
+		$this->assertSame( array(), get_post_meta( $row_id, "field-{$field_id}", false ) );
+	}
+
 	public function test_query_rows_includes_collection_metadata(): void {
 		wp_set_current_user( $this->create_user( 'author' ) );
 		$fixture = $this->create_collection_fixture( 'rowmeta', 'text' );