scope requires join and present condition

dummy@example.com (thorstenhirsch) — Thu, 03 Jan 2013 22:50:58 +0000

Let's say my model looks like this

* Guest
* Ride (n:m)
* Taxi

The rides have time_from/time_to attributes, so the association between Guest and Taxi is valid within a time frame. Now I want to create a scope in Guest that selects me only those guests that are currently riding in a taxi, which means that guest.taxi.present? = true ...but only if Time.zone.now is between ride.time_from and ride.time_to (or Time.zone.now is >= ride.time_from and ride.time_to is null).
I think I can go the hard way and write a long SQL statement that does 2 joins manually. But since I've defined the relationship already (guest has_many taxis, :through => :ride), I think there must be a better way... could you help me please?

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scope requires join and present condition