def _expected_cols(expected_attrs):
    if not expected_attrs:
        return expected_attrs

    simple_cols = [attr for attr in expected_attrs
                   if attr in _INSTANCE_OPTIONAL_JOINED_FIELDS]

    complex_cols = ['extra.%s' % field
                    for field in _INSTANCE_EXTRA_FIELDS
                    if field in expected_attrs]
    if complex_cols:
        simple_cols.append('extra')
    simple_cols = [x for x in simple_cols if x not in _INSTANCE_EXTRA_FIELDS]
    return simple_cols + complex_cols
Return expected_attrs that are columns needing joining.
dataset/ETHPy150Open BU-NU-CLOUD-SP16/Trusted-Platform-Module-nova/nova/objects/instance.py/_expected_cols