#24793: Subtracting DateTime fields in a query expression should use timediff
-------------------------------------+-------------------------------------
Reporter: sparkyb | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: 1.8
(models, ORM) |
Severity: Normal | Resolution:
Keywords: datetime duration | Triage Stage: Accepted
expression |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by jarshwah):
* cc: josh.smeaton@… (added)
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
* type: Uncategorized => New feature
* stage: Unreviewed => Accepted
Comment:
Backends that don't have native duration types (mysql and sqlite) do not
have support for `date - date` annotations. This is guarded by the
`has_native_duration_field` flag in tests.
You can implement your own expression that uses timediff though:
{{{
class TimeDiff(Func):
function = 'timediff'
output_field = TimeField()
# you could also implement __init__ to enforce only two date fields
Event.objects.annotate(duration=TimeDiff('end_time', 'start_time')
}}}
Note that I've made the output_field a TimeField rather than a
DurationField. MySQL returns Time as the result of `date-date`, not a
duration/interval.
Considering that adding/subtracting dates is relatively common, I wouldn't
mind seeing better support for mysql and sqlite if it's possible. So, I'm
accepting on that basis. If someone is able to provide a patch that makes
`F('date') - F('otherdate')` work consistently on all backends, then we'd
probably accept that.
--
Ticket URL: <https://code.djangoproject.com/ticket/24793#comment:1>
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