Returns a generator that accesses each chunk in ascending order

        Parameters
        ----------
        symbol: str
            the symbol for the given item in the DB
        chunk_range: None, or a range object
            allows you to subset the chunks by range

        Returns
        -------
        generator

def iterator(self, symbol, chunk_range=None):
        """
        Returns a generator that accesses each chunk in ascending order

        Parameters
        ----------
        symbol: str
            the symbol for the given item in the DB
        chunk_range: None, or a range object
            allows you to subset the chunks by range

        Returns
        -------
        generator
        """
        sym = self._get_symbol_info(symbol)
        if not sym:
            raise NoDataFoundException("Symbol does not exist.")

        c = CHUNKER_MAP[sym[CHUNKER]]

        for chunk in list(self.get_chunk_ranges(symbol, chunk_range=chunk_range)):
            yield self.read(symbol, chunk_range=c.to_range(chunk[0], chunk[1]))