param mixed  $value     Updated value.
	 * @return bool|WP_Error True if the meta field was updated, WP_Error otherwise.
	 */
	protected function update_meta_value( $object_id, $meta_key, $name, $value ) {
		$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() )
			);
		}

		$meta_key   = wp_slash( $meta_key );
		$meta_value = wp_slash( $value );

		// Do the exact same check for a duplicate value as in update_metadata() to avoid update_metadata() returning false.
		$old_value = get_metadata( $meta_type, $object_id, $meta_key );

		if ( 1 === count( $old_value ) ) {
			if ( $old_value[0] === $meta_value ) {
				return true;
			}
		}

		if ( ! update_metadata( $meta_type, $object_id, $meta_key, $meta_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;
	}

	/**
	 * Retrieves all the registered meta fields.
	 *
	 * @since 4.7.0
	 *
	 * @return array Registered fields.
	 */
	protected function get_registered_fields() {
		$registered = array();

		$meta_type    = $this->get_meta_type();
		$meta_subtype = $this->get_meta_subtype();

		$meta_keys = get_registered_meta_keys( $meta_type );
		if ( ! empty( $meta_subtype ) ) {
			$meta_keys = array_merge( $meta_keys, get_registered_meta_keys( $meta_type, $meta_subtype ) );
		}

		foreach ( $meta_keys as $name => $args ) {
			if ( empty( $args['show_in_rest'] ) ) {
				continue;
			}

			$rest_args = array();

			if ( is_array( $args['show_in_rest'] ) ) {
				$rest_args = $args['show_in_rest'];
			}

			$default_args = array(
				'name'             => $name,
				'single'           => $args['single'],
				'type'             => ! empty( $args['type'] ) ? $args['type'] : null,
				'schema'           => array(),
				'prepare_callback' => array( $this, 'prepare_value' ),
			);

			$default_schema = array(
				'type'        => $default_args['type'],
				'description' => empty( $args['description'] ) ? '' : $args['description'],
				'default'     => isset( $args['default'] ) ? $args['default'] : null,
			);

			$rest_args = array_merge( $default_args, $rest_args );
			$rest_args['schema'] = array_merge( $default_schema, $rest_args['schema'] );

			$type = ! empty( $rest_args['type'] ) ? $rest_args['type'] : null;
			$type = ! empty( $rest_args['schema']['type'] ) ? $rest_args['schema']['type'] : $type;

			if ( ! in_array( $type, array( 'string', 'boolean', 'integer', 'number' ) ) ) {
				continue;
			}

			if ( empty( $rest_args['single'] ) ) {
				$rest_args['schema']['items'] = array(
					'type' => $rest_args['type'],
				);
				$rest_args['schema']['type'] = 'array';
			}

			$registered[ $name ] = $rest_args;
		}

		return $registered;
	}

	/**
	 * Retrieves the object's meta schema, conforming to JSON Schema.
	 *
	 * @since 4.7.0
	 *
	 * @return array Field schema data.
	 */
	public function get_field_schema() {
		$fields = $this->get_registered_fields();

		$schema = array(
			'description' => __( 'Meta fields.' ),
			'type'        => 'object',
			'context'     => array( 'view', 'edit' ),
			'properties'  => array(),
			'arg_options' => array(
				'sanitize_callback' => null,
				'validate_callback' => array( $this, 'check_meta_is_array' ),
			),
		);

		foreach ( $fields as $args ) {
			$schema['properties'][ $args['name'] ] = $args['schema'];
		}

		return $schema;
	}

	/**
	 * Prepares a meta value for output.
	 *
	 * Default preparation for meta fields. Override by passing the
	 * `prepare_callback` in your `show_in_rest` options.
	 *
	 * @since 4.7.0
	 *
	 * @param mixed           $value   Meta value from the database.
	 * @param WP_REST_Request $request Request object.
	 * @param array           $args    REST-specific options for the meta key.
	 * @return mixed Value prepared for output. If a non-JsonSerializable object, null.
	 */
	public static function prepare_value( $value, $request, $args ) {
		$type = $args['schema']['type'];

		// For multi-value fields, check the item type instead.
		if ( 'array' === $type && ! empty( $args['schema']['items']['type'] ) ) {
			$type = $args['schema']['items']['type'];
		}

		switch ( $type ) {
			case 'string':
				$value = (string) $value;
				break;
			case 'integer':
				$value = (int) $value;
				break;
			case 'number':
				$value = (float) $value;
				break;
			case 'boolean':
				$value = (bool) $value;
				break;
		}

		// Don't allow objects to be output.
		if ( is_object( $value ) && ! ( $value instanceof JsonSerializable ) ) {
			return null;
		}

		return $value;
	}

	/**
	 * Check the 'meta' value of a request is an associative array.
	 *
	 * @since 4.7.0
	 *
	 * @param  mixed           $value   The meta value submitted in the request.
	 * @param  WP_REST_Request $request Full details about the request.
	 * @param  string          $param   The parameter name.
	 * @return WP_Error|string The meta array, if valid, otherwise an error.
	 */
	public function check_meta_is_array( $value, $request, $param ) {
		if ( ! is_array( $value ) ) {
			return false;
		}

		return $value;
	}
}