#23420: Custom lookup transformers can't create DateTimeFields when USE_TZ=True
-------------------------------------+-------------------------------------
Reporter: chosak | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.7
(models, ORM) | Keywords: lookup, transform,
Severity: Normal | datetimefield
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
If you define a custom lookup transformer that converts to a
`DateTimeField`, and `USE_TZ=True`, then making a query with the transform
fails with a cryptic error.
Consider the case where you have a model that stores Unix timestamps
(seconds since 1/1/1970) as positive integers. (Granted, Django supports
DateTimeFields, but there may be other reasons why you want to store the
raw timestamp.) It might be useful to define a
[https://docs.djangoproject.com/en/dev/howto/custom-lookups/#a-simple-
transformer-example custom lookup transform] that makes use of MySQL's
[http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html
#function_from-unixtime FROM_UNIXTIME] to convert to DateTime
representation when queried.
Take for example this model:
{{{
#!python
from django.db import models
class UnixTimestamp(models.Model):
ts = models.PositiveIntegerField()
}}}
It'd be nice to be able to do something like:
{{{
#!python
from datetime import datetime, timedelta
one_week_ago = datetime.utcnow() - timedelta(days=7)
recent_ts = UnixTimestamp.objects.filter(ts__as_datetime__gt=one_week_ago)
}}}
You should be able to do this:
{{{
#!python
class PositiveIntegerDateTimeTransform(models.Transform):
lookup_name = 'as_datetime'
@property
def output_field(self):
return models.DateTimeField()
def as_sql(self, qn, connection):
lhs, params = qn.compile(self.lhs)
return 'from_unixtime({})'.format(lhs), params
models.PositiveIntegerField.register_lookup(PositiveIntegerDateTimeTransform)
}}}
But in practice the above produces output like this (see attached unit
test):
{{{
Traceback (most recent call last):
File "/dev/django/tests/custom_lookups/tests.py", line 253, in
test_datetime_output_field
UnixTimestamp.objects.filter(ts__as_datetime__gt=year_one),
File "/dev/django/django/db/models/manager.py", line 80, in
manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/dev/django/django/db/models/query.py", line 702, in filter
return self._filter_or_exclude(False, *args, **kwargs)
File "/dev/django/django/db/models/query.py", line 720, in
_filter_or_exclude
clone.query.add_q(Q(*args, **kwargs))
File "/dev/django/django/db/models/sql/query.py", line 1319, in add_q
clause, require_inner = self._add_q(where_part, self.used_aliases)
File "/dev/django/django/db/models/sql/query.py", line 1346, in _add_q
current_negated=current_negated, connector=connector)
File "/dev/django/django/db/models/sql/query.py", line 1218, in
build_filter
condition = self.build_lookup(lookups, col, value)
File "/dev/django/django/db/models/sql/query.py", line 1123, in
build_lookup
return final_lookup(lhs, rhs)
File "/dev/django/django/db/models/lookups.py", line 82, in __init__
self.rhs = self.get_prep_lookup()
File "/dev/django/django/db/models/lookups.py", line 85, in
get_prep_lookup
return self.lhs.output_field.get_prep_lookup(self.lookup_name,
self.rhs)
File "/dev/django/django/db/models/fields/__init__.py", line 1249, in
get_prep_lookup
return super(DateField, self).get_prep_lookup(lookup_type, value)
File "/dev/django/django/db/models/fields/__init__.py", line 651, in
get_prep_lookup
return self.get_prep_value(value)
File "/dev/django/django/db/models/fields/__init__.py", line 1405, in
get_prep_value
(self.model.__name__, self.name, value),
AttributeError: 'DateTimeField' object has no attribute 'model'
}}}
This error is caused by a `DateTimeField` naive timezone warning (code
[https://github.com/django/django/blob/1.7/django/db/models/fields/__init__.py#L1275
here]) assuming that the field instance is bound to a model, which it is
not in this case.
3 attachments to this ticket:
1. output_field_docs.patch : clarifies documentation of `output_field` in
`django.db.models.Transform`, which is currently wrong
2. datetime_unbound_warning.patch : modifies `DateTimeField` to warn about
naive datetimes with unbound `Field` instances.
3. datetime_custom_lookup_test.patch : test that demonstrates the above-
described scenario (MySQL only)
--
Ticket URL: <https://code.djangoproject.com/ticket/23420>
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