id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
19578	lazy prefetch_related	g00fy	nobody	"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).

"	New feature	closed	Database layer (models, ORM)	dev	Normal	wontfix	prefetch_related		Unreviewed	0	0	0	0	0	0