@classmethod
    def _check_geo_field(cls, opts, lookup):
        """
        Utility for checking the given lookup with the given model options.
        The lookup is a string either specifying the geographic field, e.g.
        'point, 'the_geom', or a related lookup on a geographic field like
        'address__point'.

        If a GeometryField exists according to the given lookup on the model
        options, it will be returned.  Otherwise returns None.
        """
        # This takes into account the situation where the lookup is a
        # lookup to a related geographic field, e.g., 'address__point'.
        field_list = lookup.split(LOOKUP_SEP)

        # Reversing so list operates like a queue of related lookups,
        # and popping the top lookup.
        field_list.reverse()
        fld_name = field_list.pop()

        try:
            geo_fld = opts.get_field(fld_name)
            # If the field list is still around, then it means that the
            # lookup was for a geometry field across a relationship --
            # thus we keep on getting the related model options and the
            # model field associated with the next field in the list
            # until there's no more left.
            while len(field_list):
                opts = geo_fld.rel.to._meta
                geo_fld = opts.get_field(field_list.pop())
        except (FieldDoesNotExist, __HOLE__):
            return False

        # Finally, make sure we got a Geographic field and return.
        if isinstance(geo_fld, GeometryField):
            return geo_fld
        else:
            return False
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.4/django/contrib/gis/db/models/sql/where.py/GeoWhereNode._check_geo_field