Mixing
Does select_related on an already queryed field perform a SQL query?
Let's assume that Car has multiple wheels and a wheel has one color.
Does the 4th line performs an SQL query ?
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
python django django-models orm django-orm
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
add a comment |
Let's assume that Car has multiple wheels and a wheel has one color.
Does the 4th line performs an SQL query ?
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
python django django-models orm django-orm
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
add a comment |
Let's assume that Car has multiple wheels and a wheel has one color.
Does the 4th line performs an SQL query ?
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
python django django-models orm django-orm
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
Let's assume that Car has multiple wheels and a wheel has one color.
Does the 4th line performs an SQL query ?
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
python django django-models orm django-orm
python django django-models orm django-orm
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
edited Nov 21 '18 at 11:34
Louis Saglio
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
asked Nov 21 '18 at 10:49
Louis SaglioLouis Saglio
13524
13524
add a comment |
add a comment |
1 Answer
1
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oldest
votes
Your question asks about select_related() but your code specifies prefetch_related() so I'll address both.
The following will hit the database once:
cars = Car.objects.select_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
The following will hit the database three times:
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
However, the following will hit the database twice:
cars = Car.objects.select_related('wheel_set').prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
https://docs.djangoproject.com/en/2.1/ref/models/querysets/#select-related
answered Nov 21 '18 at 17:25
tommytommy
3171310
Thank you for pointing out to me the absence of select related, I actually forgot a line :cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)
– Louis Saglio
Nov 22 '18 at 9:26
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Your question asks about select_related() but your code specifies prefetch_related() so I'll address both.
The following will hit the database once:
cars = Car.objects.select_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
The following will hit the database three times:
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
However, the following will hit the database twice:
cars = Car.objects.select_related('wheel_set').prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
https://docs.djangoproject.com/en/2.1/ref/models/querysets/#select-related
answered Nov 21 '18 at 17:25
tommytommy
3171310
Thank you for pointing out to me the absence of select related, I actually forgot a line :cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)
– Louis Saglio
Nov 22 '18 at 9:26
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
add a comment |
Your question asks about select_related() but your code specifies prefetch_related() so I'll address both.
The following will hit the database once:
cars = Car.objects.select_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
The following will hit the database three times:
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
However, the following will hit the database twice:
cars = Car.objects.select_related('wheel_set').prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
https://docs.djangoproject.com/en/2.1/ref/models/querysets/#select-related
answered Nov 21 '18 at 17:25
tommytommy
3171310
Thank you for pointing out to me the absence of select related, I actually forgot a line :cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)
– Louis Saglio
Nov 22 '18 at 9:26
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
add a comment |
Your question asks about select_related() but your code specifies prefetch_related() so I'll address both.
The following will hit the database once:
cars = Car.objects.select_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
The following will hit the database three times:
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
However, the following will hit the database twice:
cars = Car.objects.select_related('wheel_set').prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
https://docs.djangoproject.com/en/2.1/ref/models/querysets/#select-related
answered Nov 21 '18 at 17:25
tommytommy
3171310
Your question asks about select_related() but your code specifies prefetch_related() so I'll address both.
The following will hit the database once:
cars = Car.objects.select_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
The following will hit the database three times:
cars = Car.objects.prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
However, the following will hit the database twice:
cars = Car.objects.select_related('wheel_set').prefetch_related('wheel_set__color')
for car in cars:
for wheel in car.wheel_set:
print(wheel.color)
https://docs.djangoproject.com/en/2.1/ref/models/querysets/#select-related
answered Nov 21 '18 at 17:25
tommytommy
3171310
answered Nov 21 '18 at 17:25
tommytommy
3171310
answered Nov 21 '18 at 17:25
tommytommy
3171310
answered Nov 21 '18 at 17:25
tommytommy
3171310
3171310
Thank you for pointing out to me the absence of select related, I actually forgot a line :cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)
– Louis Saglio
Nov 22 '18 at 9:26
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
add a comment |
Thank you for pointing out to me the absence of select related, I actually forgot a line :cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)
– Louis Saglio
Nov 22 '18 at 9:26
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
Thank you for pointing out to me the absence of select related, I actually forgot a line :
cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)– Louis Saglio
Nov 22 '18 at 9:26
Thank you for pointing out to me the absence of select related, I actually forgot a line :
cars = Car.objects.prefetch_related('wheel_set__color') for car in cars: car.select_related('wheel_set') for wheel in car.wheel_set: print(wheel.color)– Louis Saglio
Nov 22 '18 at 9:26
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
I don't see why you would do that. Is your intent to minimize the number of database queries?
– tommy
Nov 28 '18 at 16:14
add a comment |
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