I agree it's a bit devious, it took me a couple of minutes to figure out it
would work for this particular use case.
Ideally there would a kwarg to Prefect that does the limiting
automatically, something like:
SELECT train.id, s.* FROM train LATERAL JOIN (
SELECT * FROM seat WHERE seat.train_id = train.id LIMIT 5
) s ON TRUE
WHERE train.id IN (1,2,3)
Le mercredi 4 mars 2015 16:27:16 UTC-5, cool-RR a écrit :
>
> Thanks Simon! That worked.
>
> I would say it's a bit devious though, and it was nice if there was a
> method that didn't use `distinct`, which has its own issues. (And can't be
> used for any number of items other than 1 in this case.)
>
> On Tue, Mar 3, 2015 at 7:15 AM, Simon Charette <[email protected]
> <javascript:>> wrote:
>
>> 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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