public function join($table, $key_one, $operator, $key_two = null)
    {
        if (is_null($key_two)) {
            $key_two = $operator;
            $operator = '=';
        }

        $this->joins .= " JOIN {$table} ON {$key_one} {$operator} {$key_two} ";

        return $this;
    }