/**
     * from a query string, prepare a mysqli statement and execute it
     * @param $query_str
     * @param $fields array of fields that will be bound to the query
     * @return the query results
     */
    private function build_and_execute( $query_str, $fields ) {
        $dbc = \appcore\config::database( $this->_database );

        $mysqli = new \mysqli( $dbc['host'], $dbc['login'], $dbc['password'], $dbc['db_name'] );

        $statement = $mysqli->prepare( $query_str );

        # build the param 1 of bind_param(), the definition string
        $bind_types = array();
        foreach ( $fields as $field ) $bind_types[] = $this->_field_types[ $field ];
        $bind_string = implode( '', $bind_types );

        # build the array of references
        $field_values = array();
        $bind_params = array();
        foreach ( $fields as $i => $v ) {
            $bind_var = 'bind_field_' . $i;
            $$bind_var = $this->_fields[ $v ];
            $bind_params[$i] = &$$bind_var;
        }
        array_unshift( $bind_params, $bind_string );
        call_user_func_array( array( $statement, 'bind_param' ), $bind_params );

        return $statement->execute();
    }