WP_REST_Controller::get_fields_for_response( WP_REST_Request $request )

	public function get_fields_for_response( $request ) {
		$schema = $this->get_item_schema();
		$fields = isset( $schema['properties'] ) ? array_keys( $schema['properties'] ) : array();
		if ( ! isset( $request['_fields'] ) ) {
			return $fields;
		}
		$requested_fields = is_array( $request['_fields'] ) ? $request['_fields'] : preg_split( '/[\s,]+/', $request['_fields'] );
		if ( 0 === count( $requested_fields ) ) {
			return $fields;
		}
		// Trim off outside whitespace from the comma delimited list.
		$requested_fields = array_map( 'trim', $requested_fields );
		// Always persist 'id', because it can be needed for add_additional_fields_to_object().
		if ( in_array( 'id', $fields, true ) ) {
			$requested_fields[] = 'id';
		}
		return array_intersect( $fields, $requested_fields );
	}