def _field_post(field):
    """Add sqlalchemy conversion helper info

    Convert aggregation -> _aggregation_fn,
    as -> _cast_to_datatype and
    default -> _coalesce_to_value"""
    if "as" in field:
        field["_cast_to_datatype"] = field.pop("as")

    if "default" in field:
        field["_coalesce_to_value"] = field.pop("default")

    return field