def to_mongo(self, prefix=None):
        """Returns a MongoDB representation of the expression.

        Args:
            prefix (None): an optional prefix to prepend to all
                :class:`ViewField` instances in the expression

        Returns:
            a MongoDB expression
        """
        if self.is_frozen:
            prefix = self._prefix

        return _do_to_mongo(self._expr, prefix)
Returns a MongoDB representation of the expression.

        Args:
            prefix (None): an optional prefix to prepend to all
                :class:`ViewField` instances in the expression

        Returns:
            a MongoDB expression
        
fiftyone/core/expressions.py