Cast crashes on SQLite when casting a Python date/datetime to Date/DateTimeField

[direx]

When trying to cast a date object to a DateField in annotate an exception is raised:

import datetime
from django.contrib.auth.models import User
from django.db import models
from django.db.models.functions import Cast

User.objects.get_or_create(username="test")
User.objects.all().annotate(today=Cast(datetime.date.today(), models.DateField()))

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/models/query.py", line 226, in __repr__
    data = list(self[:REPR_OUTPUT_SIZE + 1])
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/models/query.py", line 250, in __iter__
    self._fetch_all()
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/models/query.py", line 1118, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/models/query.py", line 62, in __iter__
    for row in compiler.results_iter(results):
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 842, in results_iter
    row = self.apply_converters(row, converters)
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 827, in apply_converters
    value = converter(value, expression, self.connection, self.query.context)
  File "/tmp/testenv/lib/python2.7/site-packages/django/db/backends/sqlite3/operations.py", line 227, in convert_datefield_value
    value = parse_date(value)
  File "/tmp/testenv/lib/python2.7/site-packages/django/utils/dateparse.py", line 61, in parse_date
    match = date_re.match(value)
TypeError: expected string or buffer

This only seems to happen with SQLite, MySQL works as expected.

FYI, this is the SQL which is generated for SQLite:

SELECT "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined", CAST(2017-10-20 AS date) AS "today" FROM "auth_user"

[Simon Charette]

Hello direx, thanks for your report!

Could you post the entire traceback of the TypeError please. Feel free to remove the parts of the file paths that contain sensitive or data.

This is probably caused by the fact SQLite stores dates a string internally. Also, I think you'd want to use annotate(today=Value(datetime.date.today())) instead.

[direx]

OK, I have updated the original text in the ticket to include the full stracktrace.

Replying to Simon Charette:

Also, I think you'd want to use annotate(today=Value(datetime.date.today())) instead.

Actually I am doing something entirely different (I need Coalesce() and Max()) and ran into this issue right there. The above code is just a minimum demonstration on how the bug can be triggered. I know that the code doesn't actually make sense, but I did not want to make the generated query more complex than it has to be for the example.

[Simon Charette]

Well aggregation [won't work on temporal data times as documented](​https://docs.djangoproject.com/en/1.11/ref/models/querysets/#aggregation-functions).

SQLite can’t handle aggregation on date/time fields out of the box. This is because there are no native date/time fields in SQLite and Django currently emulates these features using a text field. Attempts to use aggregation on date/time fields in SQLite will raise NotImplementedError.

You might hit another wall there.

[Simon Charette]

I managed to reproduce locally but I'm not sure what the resolution should be here. I feel like this is an abuse of the Cast function as it wasn't meant to be used with Python primitives, Value should be used instead.

[direx]

Replying to Simon Charette:

I managed to reproduce locally but I'm not sure what the resolution should be here. I feel like this is an abuse of the Cast function as it wasn't meant to be used with Python primitives, Value should be used instead.

In the Django docs it is expicitely stated that Cast is required in combination with Python types and Coalesce. It says:

A Python value passed to Coalesce on MySQL may be converted to an incorrect type unless explicitly cast to the correct database type

See the example right here:

​https://docs.djangoproject.com/en/1.11/ref/models/database-functions/#coalesce

The code in the given example does not work on SQLite and triggers this very bug, so I think this is a valid issue for all current Django versions.

[Mariusz Felisiak]

The main issue is that SQLite doesn't have DATE or DATETIME datatypes, therefore CAST returns incorrect result:

sqlite> select CAST('2017-10-10' AS DATE);
2017

[Mariusz Felisiak]

​PR

[Mariusz Felisiak]

Tim why you marked this patch as "needs improvement"? You didn't leave any comments or suggestions on PR.

[Tim Martin]

Sorry, I should have put a comment in here. The Oracle build on the pull request is failing.

[Mariusz Felisiak]

Yes, but it's not related with this PR (see ​PR9468 and ​discussion).

[GitHub <noreply@…>]

In 4420761e:

Fixed #28727 -- Fixed Cast crash on SQLite when casting a Python date/datetime to Date/DateTimeField.