Custom script ObjectVar default to first available option?

[llamafilm]

I'd like to make an ObjectVar which defaults to the first available option. I don't know the available options in advance because query_params is based on another variable. Is this possible? Example code below:

    pdu = ObjectVar(
        model=Device,
        query_params={"role": "power", "location_id": "$location"},
    )

    outlet = ObjectVar(
        model=PowerOutlet,
        query_params={"device_id": "$pdu", "occupied": False},
    )