Opened 16 years ago

Closed 12 years ago

#10466 closed Cleanup/optimization (duplicate)

Document that functions in QuerySets' parameters are eagerly evaluated.

Reported by: liangent Owned by: nobody
Component: Documentation Version: dev
Severity: Normal Keywords:
Cc: liangent@… Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

if i write code like this:

qs = MyModel.objects.filter(dt__lte=datetime.datetime.now)

and execute it at time A, then save qs somewhere.

later, at time B, i call for obj in qs to get the object list. as the documentation says, database query is executed at time B. however, it seems that the value of dt__lte was fetched at time A (ie. datetime.datetime.now was evaluated at time A), and i think datetime.datetime.now should be evaluated at time B (if i wrote

qs = MyModel.objects.filter(dt__lte=datetime.datetime.now())

, it should be evaluated at time A). it seems to go against "QuerySets are lazy", doesn't it?

Change History (9)

comment:1 by Malcolm Tredinnick, 16 years ago

Component: Database layer (models, ORM)Documentation
Resolution: fixed
Status: newclosed
Summary: functions as QuerySets' parameters are not lazyDocuemtn that functions in QuerySets' parameters are eagerly evaluated.
Triage Stage: UnreviewedAccepted

I'm going to call this one invalid, but it does need better documentation.

The queryset's evaluation against the database is lazy. The parameter evaluation (unless it's a database query) is done when you create the queryset. It's fairly complicated to arrange for this to be otherwise in all cases and rather than mess around with when it does and doesn't work, we always have eager parameter evaluation.

I will add some documentation, but the behaviour isn't going to change. Sorry about that. :-)

Changing the title to reflect how we'll resolve this.

comment:2 by liangent, 16 years ago

Summary: Docuemtn that functions in QuerySets' parameters are eagerly evaluated.Document that functions in QuerySets' parameters are eagerly evaluated.

comment:3 by Malcolm Tredinnick, 16 years ago

Resolution: fixed
Status: closedreopened

Not quite sure why I closed this, since I didn't update the documentation.

comment:4 by anonymous, 15 years ago

This has bitten me before as well. You might also want to add that if you pass that QuerySet as a parameter to a view in your url config, the QuerySet will have the datetime of server start, since it is only evaluated once.

And also add a way to work around this limitation? (for both the original problem and my url conf problem)

comment:5 by Chris Beaven, 14 years ago

Severity: Normal
Type: Cleanup/optimization

comment:6 by Aymeric Augustin, 13 years ago

UI/UX: unset

Change UI/UX from NULL to False.

comment:7 by Aymeric Augustin, 13 years ago

Easy pickings: unset

Change Easy pickings from NULL to False.

comment:8 by Aymeric Augustin, 12 years ago

Status: reopenednew

comment:9 by Baptiste Mispelon, 12 years ago

Resolution: duplicate
Status: newclosed

I'm closing this as a duplicate of #20241.

Even though this ticket is much older, the other one has some explanations as to why fixing this bug is tricky.

Personally, I think this feature should be removed since it doesn't the only thing it does (from what I understand) is basically save the user from typing a pair of parentheses.

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