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.



git-svn-id: https://develop.svn.wordpress.org/trunk@48402 602fd350-edb4-49c9-b593-d223f7449a82

src/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;
+}

src/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'];

tests/phpunit/tests/meta/registerMeta.php

@@ -504,6 +504,73 @@ class Tests_Meta_Register_Meta extends WP_UnitTestCase {
 		$this->assertSame( 'even', $subtype_for_4 );
 	}
 
+	/**
+	 * @ticket 43941
+	 * @dataProvider data_get_default_data
+	 */
+	public function test_get_default_value( $args, $single, $expected ) {
+
+		$object_type = 'post';
+		$meta_key    = 'registered_key1';
+		register_meta(
+			$object_type,
+			$meta_key,
+			$args
+		);
+
+		$object_property_name = $object_type . '_id';
+		$object_id            = self::$$object_property_name;
+		$default_value        = get_metadata_default( $object_type, $meta_key, $single, $object_id );
+		$this->assertSame( $default_value, $expected );
+
+		// Check for default value.
+		$value = get_metadata( $object_type, $object_id, $meta_key, $single );
+		$this->assertSame( $value, $expected );
+
+		// Set value to check default is not being returned by mistake.
+		$meta_value = 'dibble';
+		update_metadata( $object_type, $object_id, $meta_key, $meta_value );
+		$value = get_metadata( $object_type, $object_id, $meta_key, true );
+		$this->assertSame( $value, $meta_value );
+
+		// Delete meta, make sure the default is returned.
+		delete_metadata( $object_type, $object_id, $meta_key );
+		$value = get_metadata( $object_type, $object_id, $meta_key, $single );
+		$this->assertSame( $value, $expected );
+
+		// Set other meta key, to make sure other keys are not effects.
+		$meta_value = 'hibble';
+		$meta_key   = 'unregistered_key1';
+		$value      = get_metadata( $object_type, $object_id, $meta_key, true );
+		$this->assertSame( $value, '' );
+		update_metadata( $object_type, $object_id, $meta_key, $meta_value );
+		$value = get_metadata( $object_type, $object_id, $meta_key, true );
+		$this->assertSame( $value, $meta_value );
+
+	}
+
+	/**
+	 * @ticket 43941
+	 * @dataProvider data_get_invalid_default_data
+	 */
+	public function test_get_invalid_default_value( $args, $single, $expected ) {
+		$this->setExpectedIncorrectUsage( 'register_meta' );
+		$object_type = 'post';
+		$meta_key    = 'registered_key1';
+		$register    = register_meta(
+			$object_type,
+			$meta_key,
+			$args
+		);
+
+		$this->assertFalse( $register );
+
+		$object_property_name = $object_type . '_id';
+		$object_id            = self::$$object_property_name;
+		$default_value        = get_metadata_default( $object_type, $meta_key, $single, $object_id );
+		$this->assertSame( $default_value, $expected );
+	}
+
 	public function filter_get_object_subtype_for_customtype( $subtype, $object_id ) {
 		if ( 1 === ( $object_id % 2 ) ) {
 			return 'odd';
@@ -512,6 +579,501 @@ class Tests_Meta_Register_Meta extends WP_UnitTestCase {
 		return 'even';
 	}
 
+	public function data_get_default_data() {
+		return array(
+			'single string key with single ask '          => array(
+				array(
+					'single'  => true,
+					'default' => 'wibble',
+				),
+				true,
+				'wibble',
+			),
+			'single string key with multiple ask'         => array(
+				array(
+					'single'  => true,
+					'default' => 'wibble',
+				),
+				false,
+				array( 'wibble' ),
+			),
+			'multiple string key with single ask'         => array(
+				array(
+					'single'  => false,
+					'default' => 'wibble',
+				),
+				true,
+				'wibble',
+			),
+			'multiple string key with multiple ask'       => array(
+				array(
+					'single'  => false,
+					'default' => 'wibble',
+				),
+				false,
+				array( 'wibble' ),
+			),
+			'single array key with multiple ask'          => array(
+				array(
+					'single'       => true,
+					'type'         => 'array',
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'default'      => array( 'wibble' ),
+				),
+				false,
+				array( array( 'wibble' ) ),
+			),
+			'single string key with single ask for sub type' => array(
+				array(
+					'single'         => true,
+					'object_subtype' => 'page',
+					'default'        => 'wibble',
+				),
+				true,
+				'wibble',
+			),
+			'single string key with multiple ask for sub type' => array(
+				array(
+					'single'         => true,
+					'object_subtype' => 'page',
+					'default'        => 'wibble',
+				),
+				false,
+				array( 'wibble' ),
+			),
+			'single array key with multiple ask for sub type' => array(
+				array(
+					'single'         => true,
+					'object_subtype' => 'page',
+					'show_in_rest'   => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'default'        => array( 'wibble' ),
+				),
+				false,
+				array( array( 'wibble' ) ),
+			),
+
+			// types
+			'single object key with single ask'           => array(
+				array(
+					'single'       => true,
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				true,
+				array( 'wibble' => 'dibble' ),
+			),
+			'single object key with multiple ask'         => array(
+				array(
+					'single'       => true,
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				false,
+				array( array( 'wibble' => 'dibble' ) ),
+			),
+			'multiple object key with single ask'         => array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'single'       => false,
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				true,
+				array( 'wibble' => 'dibble' ),
+			),
+			'multiple object key with multiple ask'       => array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'single'       => false,
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				false,
+				array( array( 'wibble' => 'dibble' ) ),
+			),
+			'single array key with multiple ask part two' => array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => true,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				false,
+				array( array( 'dibble' ) ),
+			),
+			'multiple array with multiple ask'            => array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => false,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				false,
+				array( array( 'dibble' ) ),
+			),
+			'single array with single ask'                => array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => true,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				true,
+				array( 'dibble' ),
+			),
+
+			'multiple array with single ask'              => array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => false,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				true,
+				array( 'dibble' ),
+			),
+
+			'single boolean with single ask'              => array(
+				array(
+					'single'  => true,
+					'type'    => 'boolean',
+					'default' => true,
+				),
+				true,
+				true,
+			),
+			'multiple boolean with single ask'            => array(
+				array(
+					'single'  => false,
+					'type'    => 'boolean',
+					'default' => true,
+				),
+				true,
+				true,
+			),
+			'single boolean with multiple ask'            => array(
+				array(
+					'single'  => true,
+					'type'    => 'boolean',
+					'default' => true,
+				),
+				false,
+				array( true ),
+			),
+			'multiple boolean with multiple ask'          => array(
+				array(
+					'single'  => false,
+					'type'    => 'boolean',
+					'default' => true,
+				),
+				false,
+				array( true ),
+			),
+
+			'single integer with single ask'              => array(
+				array(
+					'single'  => true,
+					'type'    => 'integer',
+					'default' => 123,
+				),
+				true,
+				123,
+			),
+			'multiple integer with single ask'            => array(
+				array(
+					'single'  => false,
+					'type'    => 'integer',
+					'default' => 123,
+				),
+				true,
+				123,
+			),
+			'single integer with multiple ask'            => array(
+				array(
+					'single'  => true,
+					'type'    => 'integer',
+					'default' => 123,
+				),
+				false,
+				array( 123 ),
+			),
+			'multiple integer with multiple ask'          => array(
+				array(
+					'single'  => false,
+					'type'    => 'integer',
+					'default' => 123,
+				),
+				false,
+				array( 123 ),
+			),
+			'single array of objects with multiple ask'   => array(
+				array(
+					'type'         => 'array',
+					'single'       => true,
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type'       => 'object',
+								'properties' => array(
+									'name' => array(
+										'type' => 'string',
+									),
+								),
+							),
+						),
+					),
+					'default'      => array(
+						array(
+							'name' => 'Kirk',
+						),
+					),
+				),
+				false,
+				array(
+					array(
+						array(
+							'name' => 'Kirk',
+						),
+					),
+				),
+			),
+		);
+	}
+
+	public function data_get_invalid_default_data() {
+		return array(
+			array(
+				array(
+					'single'  => true,
+					'type'    => 'boolean',
+					'default' => 123,
+				),
+				true,
+				'',
+			),
+			array(
+				array(
+					'single'  => false,
+					'type'    => 'boolean',
+					'default' => 123,
+				),
+				true,
+				'',
+			),
+			array(
+				array(
+					'single'  => true,
+					'type'    => 'boolean',
+					'default' => 123,
+				),
+				false,
+				array(),
+			),
+			array(
+				array(
+					'single'  => false,
+					'type'    => 'boolean',
+					'default' => 123,
+				),
+				false,
+				array(),
+			),
+
+			array(
+				array(
+					'single'  => true,
+					'type'    => 'integer',
+					'default' => 'wibble',
+				),
+				true,
+				'',
+			),
+			array(
+				array(
+					'single'  => false,
+					'type'    => 'integer',
+					'default' => 'wibble',
+				),
+				true,
+				'',
+			),
+			array(
+				array(
+					'single'  => true,
+					'type'    => 'integer',
+					'default' => 'wibble',
+				),
+				false,
+				array(),
+			),
+			array(
+				array(
+					'single'  => false,
+					'type'    => 'integer',
+					'default' => 'wibble',
+				),
+				false,
+				array(),
+			),
+			array(
+				array(
+					'single'  => false,
+					'type'    => 'integer',
+					'default' => array( 123, 'wibble' ),
+				),
+				false,
+				array(),
+			),
+			array(
+				array(
+					'single'  => false,
+					'type'    => 'integer',
+					'default' => array( 123, array() ),
+				),
+				false,
+				array(),
+			),
+			array(
+				array(
+					'single'       => false,
+					'type'         => 'array',
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'default'      => array( array( 123, 456 ), array( 'string' ) ),
+				),
+				false,
+				array(),
+			),
+			array(
+				array(
+					'single'       => true,
+					'type'         => 'array',
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'default'      => array( array( 123, 456 ), array( 'string' ) ),
+				),
+				true,
+				'',
+			),
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'my_prop'          => array(
+									'type' => 'string',
+								),
+								'my_required_prop' => array(
+									'type' => 'string',
+								),
+							),
+							'required'   => array( 'my_required_prop' ),
+						),
+					),
+					'type'         => 'object',
+					'single'       => true,
+					'default'      => array( 'my_prop' => 'hibble' ),
+				),
+				true,
+				'',
+			),
+		);
+	}
+
 	public function data_get_types_and_subtypes() {
 		return array(
 			array( 'post', 'page' ),

tests/phpunit/tests/rest-api/rest-post-meta-fields.php

@@ -234,6 +234,17 @@ class WP_Test_REST_Post_Meta_Fields extends WP_Test_REST_TestCase {
 			)
 		);
 
+		register_post_meta(
+			'post',
+			'with_default',
+			array(
+				'type'         => 'string',
+				'single'       => true,
+				'show_in_rest' => true,
+				'default'      => 'Goodnight Moon',
+			)
+		);
+
 		/** @var WP_REST_Server $wp_rest_server */
 		global $wp_rest_server;
 		$wp_rest_server = new Spy_REST_Server;
@@ -2785,6 +2796,206 @@ class WP_Test_REST_Post_Meta_Fields extends WP_Test_REST_TestCase {
 		$this->assertNotFalse( get_metadata_by_mid( 'post', $mid2 ) );
 	}
 
+	/**
+	 * @ticket 43941
+	 * @dataProvider data_get_default_data
+	 */
+	public function test_get_default_value( $args, $expected ) {
+		$object_type = 'post';
+		$meta_key    = 'registered_key1';
+		$registered  = register_meta(
+			$object_type,
+			$meta_key,
+			$args
+		);
+
+		$this->assertTrue( $registered );
+
+		// Check for default value.
+		$request  = new WP_REST_Request( 'GET', sprintf( '/wp/v2/posts/%d', self::$post_id ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertEquals( 200, $response->get_status() );
+
+		$data = $response->get_data();
+		$this->assertArrayHasKey( 'meta', $data );
+
+		$meta = (array) $data['meta'];
+		$this->assertArrayHasKey( $meta_key, $meta );
+		$this->assertEquals( $expected, $meta[ $meta_key ] );
+	}
+
+	public function data_get_default_data() {
+		return array(
+			array(
+				array(
+					'show_in_rest' => true,
+					'single'       => true,
+					'default'      => 'wibble',
+				),
+				'wibble',
+			),
+			array(
+				array(
+					'show_in_rest' => true,
+					'single'       => false,
+					'default'      => 'wibble',
+				),
+				array( 'wibble' ),
+			),
+			array(
+				array(
+					'show_in_rest'   => true,
+					'single'         => true,
+					'object_subtype' => 'post',
+					'default'        => 'wibble',
+				),
+				'wibble',
+			),
+			array(
+				array(
+					'show_in_rest'   => true,
+					'single'         => false,
+					'object_subtype' => 'post',
+					'default'        => 'wibble',
+				),
+				array( 'wibble' ),
+			),
+			array(
+				array(
+					'single'       => true,
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				array( 'wibble' => 'dibble' ),
+			),
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'single'       => false,
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				array( array( 'wibble' => 'dibble' ) ),
+			),
+
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => true,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				array( 'dibble' ),
+			),
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => false,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				array( array( 'dibble' ) ),
+			),
+			'array of objects' => array(
+				array(
+					'type'         => 'array',
+					'single'       => true,
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type'       => 'object',
+								'properties' => array(
+									'name' => array(
+										'type' => 'string',
+									),
+								),
+							),
+						),
+					),
+					'default'      => array(
+						array(
+							'name' => 'Kirk',
+						),
+					),
+				),
+				array(
+					array(
+						'name' => 'Kirk',
+					),
+				),
+			),
+		);
+	}
+
+	/**
+	 * @ticket 43941
+	 */
+	public function test_set_default_in_schema() {
+		register_post_meta(
+			'post',
+			'greeting',
+			array(
+				'type'         => 'string',
+				'single'       => true,
+				'show_in_rest' => array(
+					'schema' => array(
+						'default' => 'Hello World',
+					),
+				),
+			)
+		);
+
+		$response = rest_do_request( '/wp/v2/posts/' . self::$post_id );
+		$this->assertEquals( 'Hello World', $response->get_data()['meta']['greeting'] );
+	}
+
+	/**
+	 * @ticket 43941
+	 */
+	public function test_default_is_added_to_schema() {
+		$request  = new WP_REST_Request( 'OPTIONS', '/wp/v2/posts' );
+		$response = rest_do_request( $request );
+
+		$schema = $response->get_data()['schema']['properties']['meta']['properties']['with_default'];
+		$this->assertArrayHasKey( 'default', $schema );
+		$this->assertEquals( 'Goodnight Moon', $schema['default'] );
+	}
+
 	/**
 	 * Internal function used to disable an insert query which
 	 * will trigger a wpdb error for testing purposes.

tests/phpunit/tests/rest-api/rest-term-meta-fields.php

@@ -1255,6 +1255,36 @@ class WP_Test_REST_Term_Meta_Fields extends WP_Test_REST_TestCase {
 		return $data;
 	}
 
+	/**
+	 * @ticket 43941
+	 */
+	public function test_get_default_value() {
+		$meta_key = 'registered_key1';
+		register_term_meta(
+			'category',
+			$meta_key,
+			array(
+				'single'       => true,
+				'type'         => 'string',
+				'default'      => 'Goodbye',
+				'show_in_rest' => true,
+			)
+		);
+
+		// Check for default value.
+		$request  = new WP_REST_Request( 'GET', sprintf( '/wp/v2/categories/%d', self::$category_id ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertEquals( 200, $response->get_status() );
+
+		$data = $response->get_data();
+		$this->assertArrayHasKey( 'meta', $data );
+
+		$meta = (array) $data['meta'];
+		$this->assertArrayHasKey( $meta_key, $meta );
+		$this->assertSame( 'Goodbye', $meta[ $meta_key ] );
+	}
+
 	/**
 	 * Internal function used to disable an insert query which
 	 * will trigger a wpdb error for testing purposes.

tests/phpunit/tests/rest-api/rest-users-controller.php

@@ -2890,6 +2890,125 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		$this->assertErrorResponse( 'rest_user_invalid_id', $response, 404 );
 	}
 
+	/**
+	 * @ticket 43941
+	 * @dataProvider data_get_default_data
+	 */
+	public function test_get_default_value( $args, $expected ) {
+		wp_set_current_user( self::$user );
+
+		$object_type = 'user';
+		$meta_key    = 'registered_key1';
+		register_meta(
+			$object_type,
+			$meta_key,
+			$args
+		);
+
+		// Check for default value.
+		$request  = new WP_REST_Request( 'GET', sprintf( '/wp/v2/users/%d', self::$user ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertEquals( 200, $response->get_status() );
+
+		$data = $response->get_data();
+		$this->assertArrayHasKey( 'meta', $data );
+
+		$meta = (array) $data['meta'];
+		$this->assertArrayHasKey( $meta_key, $meta );
+		$this->assertEquals( $expected, $meta[ $meta_key ] );
+	}
+
+	public function data_get_default_data() {
+		return array(
+			array(
+				array(
+					'show_in_rest' => true,
+					'single'       => true,
+					'default'      => 'wibble',
+				),
+				'wibble',
+			),
+			array(
+				array(
+					'show_in_rest' => true,
+					'single'       => false,
+					'default'      => 'wibble',
+				),
+				array( 'wibble' ),
+			),
+			array(
+				array(
+					'single'       => true,
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				array( 'wibble' => 'dibble' ),
+			),
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'       => 'object',
+							'properties' => array(
+								'wibble' => array(
+									'type' => 'string',
+								),
+							),
+						),
+					),
+					'type'         => 'object',
+					'single'       => false,
+					'default'      => array( 'wibble' => 'dibble' ),
+				),
+				array( array( 'wibble' => 'dibble' ) ),
+			),
+
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => true,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				array( 'dibble' ),
+			),
+			array(
+				array(
+					'show_in_rest' => array(
+						'schema' => array(
+							'type'  => 'array',
+							'items' => array(
+								'type' => 'string',
+							),
+						),
+					),
+					'single'       => false,
+					'type'         => 'array',
+					'default'      => array( 'dibble' ),
+				),
+				array( array( 'dibble' ) ),
+			),
+		);
+	}
+
 	public function additional_field_get_callback( $object ) {
 		return get_user_meta( $object['id'], 'my_custom_int', true );
 	}