Including the test for seat capacity in the validate method blocks editing of any seats on a full flight. Better would be to use validate_on_create:

class Seat < ActiveRecord::Base

belongs_to :flight

def validate
if baggage > flight.baggage_allowance
errors.add_to_base("The maximum baggage weight on this flight is " \
+ flight.baggage_allowance.to_s)
end
end

def validate_on_create
if flight.seats.size >= flight.capacity
errors.add_to_base("Sorry - this flight is already fully booked!")
end
end

end

This Oracle example won't work.
You should specify database: parameter whenever you use oracle driver (instead, host: is optional).