Subtracting DateTime fields in a query expression should use timediff

[Ben Buchwald]

If I have a model with start and end DateTime fields, I'd like to be able to annotate a query with the duration. For instance, with the model

class Event(Model):
    start_time = DateTimeField()
    end_time = DateTimeField()

I want to be able to do the following query: Event.objects.annotate(duration=ExpressionWrapper(F('end_time')-F('start_time'),output_field=DurationField())). However this yields and incorrect result. At least in MySQL, the proper SQL is timediff(end_time,start_time) as duration. I have not found an alternate way to do this with Django's ORM without writing SQL.

[Josh Smeaton]

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.

[Samuel Spencer]

@jarshwah

I've got a work around that I think works for SQLite using SQLites native julianday function, and am keen to progress this as an actual solution, but the code is a little dense for me.

Would you be able to point me in the direction of where the subtract expressions are handled?

[Josh Smeaton]

Hi Lego,

I'm really sorry I've taken so long to get back to you, I'll do better in future.

Where I think you should be looking is django.db.models.expression.CombinedExpression.as_sql() method.

You would put some handling in there to check left and right hand side for two dates, and then we'd probably have some backend specific SQL that knows how to handle date(+-)date.

If you look in the as_sql method you'll see how DurationExpressions are handled. It's guarded by a feature flag (has_native_duration_field). You might want to guard by another feature flag, something like "supports_date_arithmetic". You could then create a DateExpression class that handles the backend specific way of doing date math.

Hopefully that's enough information for you to get started. Please hit me up if you need some help or clarification.

[Simon Charette]

I tried to help someone on the user mailing list with ​a similar issue with time fields.

I came up with tests that shows that all but DateField combinations work on PostgreSQL while all combinations fails on SQLite3:

from django.db import models


class DateModel(models.Model):
    start = models.DateField()
    end = models.DateField()


class TimeModel(models.Model):
    start = models.TimeField()
    end = models.TimeField()


class DateTimeModel(models.Model):
    start = models.DateTimeField()
    end = models.DateTimeField()

import datetime

from django.db.models import DurationField, ExpressionWrapper, F
from django.test import TestCase
from pytz import UTC

from . import models


class DurationTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        models.DateModel.objects.create(
            start=datetime.date(2015, 5, 10),
            end=datetime.date(2015, 5, 12),
        )
        models.TimeModel.objects.create(
            start=datetime.time(10),
            end=datetime.time(13, 30),
        )
        models.DateTimeModel.objects.create(
            start=datetime.datetime(2015, 5, 10, 10, 00, tzinfo=UTC),
            end=datetime.datetime(2015, 5, 12, 13, 30, tzinfo=UTC),
        )

    def test_timemodel(self):
        self.assertEqual(
            models.TimeModel.objects.annotate(duration=ExpressionWrapper(
                F('end') - F('start'), output_field=DurationField()
            )).get().duration, datetime.timedelta(hours=3, minutes=30)
        )

    def test_datemodel(self):
        self.assertEqual(
            models.DateModel.objects.annotate(duration=ExpressionWrapper(
                F('end') - F('start'), output_field=DurationField()
            )).get().duration, datetime.timedelta(days=2)
        )

    def test_datetimemodel(self):
        self.assertEqual(
            models.DateTimeModel.objects.annotate(duration=ExpressionWrapper(
                F('end') - F('start'), output_field=DurationField()
            )).get().duration, datetime.timedelta(days=2, hours=3, minutes=30)
        )

PostgreSQL failure:

======================================================================
FAIL: test_datemodel (duration_expression.tests.DurationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/simon/workspace/ticketing/duration_expression/tests.py", line 40, in test_datemodel
    )).get().duration, timedelta(days=2)
AssertionError: 2 != datetime.timedelta(2)

----------------------------------------------------------------------

SQLite3 failures:

======================================================================
FAIL: test_datemodel (duration_expression.tests.DurationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/simon/workspace/ticketing/duration_expression/tests.py", line 37, in test_datemodel
    )).get().duration, datetime.timedelta(days=2)
AssertionError: datetime.timedelta(0) != datetime.timedelta(2)

======================================================================
FAIL: test_datetimemodel (duration_expression.tests.DurationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/simon/workspace/ticketing/duration_expression/tests.py", line 44, in test_datetimemodel
    )).get().duration, datetime.timedelta(days=2, hours=3, minutes=30)
AssertionError: datetime.timedelta(0) != datetime.timedelta(2, 12600)

======================================================================
FAIL: test_timemodel (duration_expression.tests.DurationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/simon/workspace/ticketing/duration_expression/tests.py", line 30, in test_timemodel
    )).get().duration, datetime.timedelta(hours=3, minutes=30)
AssertionError: datetime.timedelta(0, 0, 3) != datetime.timedelta(0, 12600)

----------------------------------------------------------------------

[Simon Charette]

​PR

[Tim Graham]

Needs a mention in the release notes for third-party database backends.

[Simon Charette <charette.s@…>]

In 766afc22:

Fixed #24793 -- Unified temporal difference support.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 088a6fa:

Refs #24793 -- Removed bogus connection argument from SQLCompiler.compile() calls.

The method doesn't expect a connection object to be passed as its second
argument.