#19578: lazy prefetch_related
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Reporter: g00fy | Owner: nobody
Type: New feature | Status: new
Component: Database layer (models, ORM) | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | prefetch_related
Easy pickings: 0 | Has patch: 0
| UI/UX: 0
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I suggest, that prefetch_related should be lazy. What I mean is, it would
only execute aditional query, when related model needs to be accessed.
Example:
{{{
# this would do just one query - prefetch_related is lazy
pizzas = Pizza.objects.all().prefetch_related('toppigns')
for pizza in pizzas:
print pizza.name
}}}
{{{
# Proposal A
# now we want to access the related objects,
# 1 aditional query (intermediate m2m table) is executed
# not 2 since, we only need the id of the related object (which is already
in the m2m table)
# the 2nd query would result in a set of 'lazy' objects
for pizza in pizzas:
for topping in pizza.toppings.all():
print topping.id
# now 3rd query is made
for pizza in pizzas:
for topping in pizza.toppings.all():
print topping.name
}}}
{{{
# Proposal B
# 2nd query is made (with the current implementation - m2m table join)
for pizza in pizzas:
for topping in pizza.toppings.all():
print topping.id
}}}
Usecase:
let's consider logic like this:
{{{
pizzas = Pizza.objects.all()
print len(pizzas)
if pizzas[0].name == 'pepperoni':
for pizza in pizzas:
print [topping.name for topping in pizza.toppings.all()]
else:
for pizza in pizzas:
print pizza.name
}}}
in otherwords, this would be useful, if there is some complex logic that
sometimes may require prefetch_related (and this requirement can be only
checked after the query was made - so there is no way to do the checking &
then doing prefetch_related).
--
Ticket URL: <https://code.djangoproject.com/ticket/19578>
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