'Adds a $where clause to this query.
The `code` argument must be an instance of :class:`basestring`
(:class:`str` in python 3) or :class:`~bson.code.Code`
containing a JavaScript expression. This expression will be
evaluated for each document scanned. Only those documents
for which the expression evaluates to *true* will be returned
as results. The keyword *this* refers to the object currently
being scanned.
Raises :class:`TypeError` if `code` is not an instance of
:class:`basestring` (:class:`str` in python 3). Raises
:class:`~pymongo.errors.InvalidOperation` if this
:class:`Cursor` has already been used. Only the last call to
:meth:`where` applied to a :class:`Cursor` has any effect.
:Parameters:
- `code`: JavaScript expression to use as a filter'

def where(self, code):

    self.__check_okay_to_chain()
    if (not isinstance(code, Code)):
        code = Code(code)
    self.__spec['$where'] = code
    return self
