REST API, Meta: Introduce support for default metadata values.

The `register_meta()` API now officially supports specifying a default metadata value. When `get_metadata()` is called for a meta key that does not yet exist for the object, this default value will be returned instead of an empty string.

A new function is introduced `get_metadata_raw` to retrieve the raw metadata value from the database, without applying the registered default.

Props spacedmonkey, flixos90, rmccue, kadamwhite, mnelson4, johnbillion, chrisvanpatten, TimothyBlynJacobs.
Fixes #43941.


Built from https://develop.svn.wordpress.org/trunk@48402


git-svn-id: http://core.svn.wordpress.org/trunk@48171 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/meta.php

@@ -210,8 +210,8 @@ function update_metadata( $meta_type, $object_id, $meta_key, $meta_value, $prev_
 
 	// Compare existing value to new value if no prev value given and the key exists only once.
 	if ( empty( $prev_value ) ) {
-		$old_value = get_metadata( $meta_type, $object_id, $meta_key );
+		$old_value = get_metadata_raw( $meta_type, $object_id, $meta_key );
-		if ( count( $old_value ) == 1 ) {
+		if ( is_countable( $old_value ) && count( $old_value ) === 1 ) {
 			if ( $old_value[0] === $meta_value ) {
 				return false;
 			}
@@ -485,7 +485,7 @@ function delete_metadata( $meta_type, $object_id, $meta_key, $meta_value = '', $
  * If the meta field does not exist, an empty string is returned if `$single` is true, or an empty array if it's false.
  * If there's a problem with the parameters passed to the function, boolean `false` is returned.
  *
- * @since 2.9.0
+ * @since 5.5.0
  *
  * @param string $meta_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
  *                          or any other object type with an associated meta table.
@@ -496,7 +496,7 @@ function delete_metadata( $meta_type, $object_id, $meta_key, $meta_value = '', $
  *                          This parameter has no effect if meta_key is not specified. Default false.
  * @return mixed The metadata value or array of values. See description above for further details.
  */
-function get_metadata( $meta_type, $object_id, $meta_key = '', $single = false ) {
+function get_metadata_raw( $meta_type, $object_id, $meta_key = '', $single = false ) {
 	if ( ! $meta_type || ! is_numeric( $object_id ) ) {
 		return false;
 	}
@@ -553,11 +553,74 @@ function get_metadata( $meta_type, $object_id, $meta_key = '', $single = false )
 		}
 	}
 
-	if ( $single ) {
+	return null;
-		return '';
+}
-	} else {
+
-		return array();
+/**
+ * Retrieves raw metadata for the specified object.
+ *
+ * @since 2.9.0
+ * @uses get_metadata_raw()
+ *
+ * @param string $meta_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
+ *                          or any other object type with an associated meta table.
+ * @param int    $object_id ID of the object metadata is for.
+ * @param string $meta_key  Optional. Metadata key. If not specified, retrieve all metadata for
+ *                          the specified object. Default empty.
+ * @param bool   $single    Optional. If true, return only the first value of the specified meta_key.
+ *                          This parameter has no effect if meta_key is not specified. Default false.
+ * @return mixed Single metadata value, or array of values
+ */
+function get_metadata( $meta_type, $object_id, $meta_key = '', $single = false ) {
+	$value = get_metadata_raw( $meta_type, $object_id, $meta_key, $single );
+	if ( ! is_null( $value ) ) {
+		return $value;
 	}
+
+	return get_metadata_default( $meta_type, $meta_key, $single, $object_id );
+}
+
+/**
+ * Retrieve metadata data default for the specified object.
+ *
+ * @since 5.5.0
+ *
+ * @param string $meta_type Type of object metadata is for (e.g., comment, post, term, or user).
+ * @param string $meta_key  Optional. Metadata key. If not specified, retrieve all metadata for
+ *                          the specified object.
+ * @param bool   $single    Optional, default is false.
+ *                          If true, return only the first value of the specified meta_key.
+ *                          This parameter has no effect if meta_key is not specified.
+ * @param int    $object_id Optional, default is 0.
+ *                          ID of the object metadata is for
+ * @return mixed Single metadata value, or array of values
+ */
+function get_metadata_default( $meta_type, $meta_key, $single = false, $object_id = 0 ) {
+	if ( $single ) {
+		$value = '';
+	} else {
+		$value = array();
+	}
+
+	/**
+	 * Filter the default value a specified object.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @param array|string      $value     The value should return - a single metadata value,
+	 *                                     or an array of values.
+	 * @param string            $meta_type Type of object metadata is for (e.g., comment, post, term, or user).
+	 * @param string            $meta_key  Meta key.
+	 * @param bool              $single    Whether to return only the first value of the specified $meta_key.
+	 * @param int               $object_id Object ID.
+	 */
+	$value = apply_filters( "default_{$meta_type}_metadata", $value, $meta_type, $meta_key, $single, $object_id );
+
+	if ( ! $single && ! wp_is_numeric_array( $value ) ) {
+		$value = array( $value );
+	}
+
+	return $value;
 }
 
 /**
@@ -1163,6 +1226,10 @@ function sanitize_meta( $meta_key, $meta_value, $object_type, $object_subtype =
  *                                         Valid values are 'string', 'boolean', 'integer', 'number', 'array', and 'object'.
  *     @type string     $description       A description of the data attached to this meta key.
  *     @type bool       $single            Whether the meta key has one value per object, or an array of values per object.
+ *     @type mixed      $default           The default value returned from {@see get_metadata()} if no value has been set yet.
+ *                                         When using a non-single meta key, the default value is for the first entry. In other
+ *                                         words, when calling {@see get_metadata()} with `$single` set to `false`, the default
+ *                                         value given here will be wrapped in an array.
  *     @type string     $sanitize_callback A function or method to call when sanitizing `$meta_key` data.
  *     @type string     $auth_callback     Optional. A function or method to call when performing edit_post_meta,
  *                                         add_post_meta, and delete_post_meta capability checks.
@@ -1188,6 +1255,7 @@ function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
 		'object_subtype'    => '',
 		'type'              => 'string',
 		'description'       => '',
+		'default'           => '',
 		'single'            => false,
 		'sanitize_callback' => null,
 		'auth_callback'     => null,
@@ -1225,6 +1293,7 @@ function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
 	 * @param string $meta_key    Meta key.
 	 */
 	$args = apply_filters( 'register_meta_args', $args, $defaults, $object_type, $meta_key );
+	unset( $defaults['default'] );
 	$args = wp_parse_args( $args, $defaults );
 
 	// Require an item schema when registering array meta.
@@ -1264,6 +1333,24 @@ function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
 		}
 	}
 
+	if ( array_key_exists( 'default', $args ) ) {
+		$schema = $args;
+		if ( is_array( $args['show_in_rest'] ) && isset( $args['show_in_rest']['schema'] ) ) {
+			$schema = array_merge( $schema, $args['show_in_rest']['schema'] );
+		}
+
+		$check = rest_validate_value_from_schema( $args['default'], $schema );
+		if ( is_wp_error( $check ) ) {
+			_doing_it_wrong( __FUNCTION__, __( 'When registering a default meta value the data must match the type provided.' ), '5.5.0' );
+
+			return false;
+		}
+
+		if ( ! has_filter( "default_{$object_type}_metadata", 'filter_default_metadata' ) ) {
+			add_filter( "default_{$object_type}_metadata", 'filter_default_metadata', 10, 5 );
+		}
+	}
+
 	// Global registry only contains meta keys registered with the array of arguments added in 4.6.0.
 	if ( ! $has_old_auth_cb && ! $has_old_sanitize_cb ) {
 		unset( $args['object_subtype'] );
@@ -1496,3 +1583,64 @@ function get_object_subtype( $object_type, $object_id ) {
 	 */
 	return apply_filters( "get_object_subtype_{$object_type}", $object_subtype, $object_id );
 }
+
+/**
+ * Filter into default_{$object_type}_metadata and add in default value.
+ *
+ * @since 5.5.0
+ *
+ * @param mixed  $value     Current value passed to filter.
+ * @param string $meta_type Type of object metadata is for (e.g., comment, post, term, or user).
+
+ * @param string $meta_key  Optional. Metadata key. If not specified, retrieve all metadata for
+ *                          the specified object.
+ * @param bool   $single    Optional, default is false.
+ *                          If true, return only the first value of the specified meta_key.
+ *                          This parameter has no effect if meta_key is not specified.
+ * @param int    $object_id ID of the object metadata is for
+ *
+ * @return mixed Single metadata default, or array of defaults
+ */
+function filter_default_metadata( $value, $meta_type, $meta_key, $single, $object_id ) {
+	global $wp_meta_keys;
+
+	if ( wp_installing() ) {
+		return $value;
+	}
+
+	if ( ! is_array( $wp_meta_keys ) || ! isset( $wp_meta_keys[ $meta_type ] ) ) {
+		return $value;
+	}
+
+	$defaults = array();
+	foreach ( $wp_meta_keys[ $meta_type ] as $sub_type => $meta_data ) {
+		foreach ( $meta_data as $_meta_key => $args ) {
+			if ( $_meta_key === $meta_key && array_key_exists( 'default', $args ) ) {
+				$defaults[ $sub_type ] = $args;
+			}
+		}
+	}
+
+	if ( ! $defaults ) {
+		return $value;
+	}
+
+	// If this meta type does not have sub types, then the default is keyed as an empty string.
+	if ( isset( $defaults[''] ) ) {
+		$metadata = $defaults[''];
+	} else {
+		$sub_type = get_object_subtype( $meta_type, $object_id );
+		if ( ! isset( $defaults[ $sub_type ] ) ) {
+			return $value;
+		}
+		$metadata = $defaults[ $sub_type ];
+	}
+
+	if ( $single ) {
+		$value = $metadata['default'];
+	} else {
+		$value = array( $metadata['default'] );
+	}
+
+	return $value;
+}

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

@@ -79,7 +79,6 @@ abstract class WP_REST_Meta_Fields {
 		foreach ( $fields as $meta_key => $args ) {
 			$name       = $args['name'];
 			$all_values = get_metadata( $this->get_meta_type(), $object_id, $meta_key, false );
-
 			if ( $args['single'] ) {
 				if ( empty( $all_values ) ) {
 					$value = $args['schema']['default'];

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-alpha-48401';
+$wp_version = '5.5-alpha-48402';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.