Internal method that determines if this asset/field combination
        represents a fetcher value or a regular OHLCVP lookup.

def _is_extra_source(asset, field, map):
        """
        Internal method that determines if this asset/field combination
        represents a fetcher value or a regular OHLCVP lookup.
        """
        # If we have an extra source with a column called "price", only look
        # at it if it's on something like palladium and not AAPL (since our
        # own price data always wins when dealing with assets).

        return not (field in BASE_FIELDS and
                    (isinstance(asset, (Asset, ContinuousFuture))))