Prefetching a single item
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. -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscr...@googlegroups.com. To post to this group, send email to django-users@googlegroups.com. Visit this group at http://groups.google.com/group/django-users. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/1d6fc84a-2042-40b5-98be-40bd971e706c%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
Re: Prefetching a single item
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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Re: Prefetching a single item
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 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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>
> .
>
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>
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Re: Prefetching a single item
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 > 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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>> .
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>> https://groups.google.com/d/msgid/django-users/b3fcf574-9024-46f9-9499-c6e67d2e3a4b%40googlegroups.com
>>
>>
>> .
>>
>> For more options, visit https://groups.google.com/d/optout.
>>
>
>
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