[Django] #28756: QuerySet API .extra( ) usage for FIELD + ORDER BY of queryset

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Oct 30, 2017, 10:56:49 AM10/30/17

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#28756: QuerySet API .extra( ) usage for FIELD + ORDER BY of queryset
-----------------------------------------+--------------------------------
Reporter: Brandon | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 1.11
Severity: Normal | Keywords: QuerySet.extra
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I have reoccurring situation, when in a view.py function I need a queryset
ordered by a set of identifiers. The order of those identifiers is
dependent on factors outside of my control. Essentially in my view I am
using modelformset_factory to generate a modelformset, then the usage of
that modelformset instance requires queryset. It is that final queryset
that must be ordered by the identifier whose order is externally
determined.
So I use the .extra( ) function like below:

{{{#!python
quali_results =
ToxicologyCaseQualitativeResult.objects.filter(case_id=case.id).select_related('test').order_by('test__name')

field_list =
["AMPHS","BARB","BNZ","BE","Ecstasy","METH","6AM","PCP","THC"] #Usually
populated via retrieval of user settings.
if len(field_list) > 0:
field_list = field_list + [q.test.identifier for q in quali_results]
fields = '"'+'","'.join(str(field) for field in field_list)+'"'
field_sql = "FIELD(`identifier`,"+fields+")"
quali_results = quali_results.extra(select={'field_sql' : field_sql},
order_by=['field_sql'])

quali_resultfs = QualitativeResultFormset(queryset=quali_results,
prefix='quali_results')
}}}

If there is a better way to do this, I would be happy to change.
Cheers!
--Brandon

--
Ticket URL: <https://code.djangoproject.com/ticket/28756>
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Django

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Oct 30, 2017, 11:45:25 AM10/30/17

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#28756: QuerySet API .extra( ) usage for FIELD + ORDER BY of queryset

-------------------------------------+-------------------------------------
Changes (by Tim Graham):

* status: new => closed
* resolution: => invalid
* component: Uncategorized => Database layer (models, ORM)

Comment:

I think that should be possible by writing your own
[https://docs.djangoproject.com/en/dev/ref/models/expressions/ Query
Expression] -- did you try it? If you have trouble, you can ask for help
on [TicketClosingReasons/UseSupportChannels our support channels]. If that
investigation shows that it's infeasible, please reopen the ticket with
details.