'Returns the first Geometry field encountered; or specified via the
`field_name` keyword.  The `field_name` may be a string specifying
the geometry field on this GeoQuery\'s model, or a lookup string
to a geometry field via a ForeignKey relation.'

def _geo_field(self, field_name=None):

    if (field_name is None):
        for fld in self.model._meta.fields:
            if isinstance(fld, GeometryField):
                return fld
        return False
    else:
        return GeoWhereNode._check_geo_field(self.model._meta, field_name)
