Can your seats be part of multiple trains?
Is there a reason you defined a ManyToMany from Train to Seat instead of a
ForeignKey from Seat to Train?
If your schema was defined as the latter the following could probably work
(given you only want the first Seat of each Train object):
trains =
Train.objects.prefetch_related(first_seats=Seat.objects.order_by('train',
'order').distinct('train'))
for train in trains:
first_seat = train.first_seats[0]
Else given you've explicitly defined a TrainSeat model to handle the
through relationship to store the order:
class TrainSeat(models.Model):
train = models.ForeignKey(Train, related_name='train_seats')
seat = models.ForeignKey(Seat)
order = models.PositiveIntegerField()
trains =
Train.objects.prefetch_related(first_train_seats=Prefetch('train_seats',
TrainSeat.objects.select_related('seat').order_by('train',
'order').distinct('train'))
for train in trains:
first_seat = train.first_train_seats[0].seat
Hope it helps.
Simon
Le lundi 2 mars 2015 17:23:29 UTC-5, cool-RR a écrit :
>
> Hi,
>
> Say that I have a model Train and it has a ManyToMany to model Seat. I'm
> looking at a queryset of Train and I want to do a prefetch_related on it,
> but I don't want to get all the Seat objects; I want only the first Seat
> object for each Train object. Is this possible with prefetch_related
> and/or Prefetch? How?
>
> To clarify: My goal here is to save on querysets while still being able to
> retrieve the first Seat object in each `train.seats`.
>
>
> Thanks,
> Ram.
>
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