[Django] #21422: prefetch_related does not document restriction
Django
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Reporter: pab@… | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: 1.6
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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I have just been trying to track down a performance issue in my
application using Django 1.6. The solution was to use prefetch_related(),
bit it didn't quite work as expected. The documentation was great as far
as it went and so I found the necessary function with little fuss - well
done! However it didn't warn me that prefetch_related() must be
restricted to a homogenous set of results.
In particular, I had query that needed to fetch data for objects found
through a GenericForeignKey and then a ManyToManyField. While the pre-
fetch claimed it had worked, it only actually queried the ManyToManyField
results for the first matching ContentType of the GenericForeignKey.
Looking at the code, this is a deliberate design decision, but it is not
documented anywhere I could see.
I understand why this restriction is in place and so am not asking for a
feature enhancement. Instead, I think the documentation for this function
needs to make clear that pre-fetching objects referenced by
GenericForeignKeys is only supported if the query has restricted the key
to one ContentType.
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Ticket URL: <https://code.djangoproject.com/ticket/21422>
Django <https://code.djangoproject.com/>
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Django
Reporter: pab@… | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: 1.6
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Changes (by loic84):
* cc: loic@… (added)
* needs_better_patch: => 0
* needs_docs: => 0
* needs_tests: => 0
* stage: Unreviewed => Accepted
Comment:
I can confirm that the implementation relies on the assumption that all
the retrieved objects are homogenous.
This limitation should be documented.
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Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:1>
Django
Reporter: pab@… | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: 1.6
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
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Changes (by ubernostrum):
* easy: 0 => 1
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Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:2>
Django
Reporter: pab@… | Owner:
Type: | donaldharvey
Cleanup/optimization | Status: assigned
Component: Documentation | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
Changes (by donaldharvey):
* status: new => assigned
* owner: nobody => donaldharvey
* version: 1.6 => master
* easy: 1 => 0
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Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:3>
Django
Reporter: pab@… | Owner:
Type: | donaldharvey
Cleanup/optimization | Status: assigned
Component: Documentation | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Changes (by timgraham):
* has_patch: 0 => 1
Comment:
[https://github.com/django/django/pull/5512 PR]
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Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:4>
Django
Reporter: pab@… | Owner:
Type: | Status: new
Cleanup/optimization |
Component: Documentation | Version: master
Severity: Normal | Resolution:
| checkin
Has patch: 1 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
Changes (by claudep):
* owner: donaldharvey =>
* status: assigned => new
* stage: Accepted => Ready for checkin
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Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:5>
Django
results
-------------------------------------+-------------------------------------
Reporter: pab@… | Owner:
Type: | Status: new
Cleanup/optimization |
Component: Documentation | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Ready for
| checkin
Has patch: 1 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
--
Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:6>
Django
results
-------------------------------------+-------------------------------------
Type: | Graham <timograham@…>
Cleanup/optimization | Status: closed
Component: Documentation | Version: master
Severity: Normal | Resolution: fixed
Keywords: | Triage Stage: Ready for
| checkin
Has patch: 1 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
Changes (by Tim Graham <timograham@…>):
* status: new => closed
* owner: => Tim Graham <timograham@…>
* resolution: => fixed
Comment:
In [changeset:"69239c0f968ee55cb9ca1e31323fb696db0c2aec" 69239c0f]:
{{{
#!CommitTicketReference repository=""
revision="69239c0f968ee55cb9ca1e31323fb696db0c2aec"
Fixed #21422 -- Documented that prefetch_related() results must be
homogeneous.
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:7>
Django
results
-------------------------------------+-------------------------------------
Reporter: pab@… | Owner: Tim
Type: | Graham <timograham@…>
Cleanup/optimization | Status: closed
Component: Documentation | Version: master
Severity: Normal | Resolution: fixed
Keywords: | Triage Stage: Ready for
| checkin
Has patch: 1 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
Comment (by Tim Graham <timograham@…>):
In [changeset:"683b3dff7b8f02e5d5a67e678698f420cbd018e9" 683b3df]:
{{{
#!CommitTicketReference repository=""
revision="683b3dff7b8f02e5d5a67e678698f420cbd018e9"
[1.8.x] Fixed #21422 -- Documented that prefetch_related() results must be
homogeneous.
Backport of 69239c0f968ee55cb9ca1e31323fb696db0c2aec from master
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:8>
Django
results
-------------------------------------+-------------------------------------
Reporter: pab@… | Owner: Tim
Type: | Graham <timograham@…>
Cleanup/optimization | Status: closed
Component: Documentation | Version: master
Severity: Normal | Resolution: fixed
Keywords: | Triage Stage: Ready for
| checkin
Has patch: 1 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
Comment (by Tim Graham <timograham@…>):
In [changeset:"31a51a04e899f1fa7fa77be38e3484074d930369" 31a51a04]:
{{{
#!CommitTicketReference repository=""
revision="31a51a04e899f1fa7fa77be38e3484074d930369"
[1.9.x] Fixed #21422 -- Documented that prefetch_related() results must be
homogeneous.
Backport of 69239c0f968ee55cb9ca1e31323fb696db0c2aec from master
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/21422#comment:9>