#29852 closed Bug (wontfix)
Infinite migrations when using SimpleLazyObject in field arguments
Reported by: | Javier Buzzi | Owned by: | nobody |
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Component: | Migrations | Version: | dev |
Severity: | Normal | Keywords: | |
Cc: | Triage Stage: | Accepted | |
Has patch: | yes | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | yes |
Easy pickings: | no | UI/UX: | no |
Description (last modified by )
from django.db import models from django.utils.functional import SimpleLazyObject from django.core.validators import MinValueValidator import datetime # Create your models here. class Thing(models.Model): day = models.DateTimeField(validators=[MinValueValidator(SimpleLazyObject(datetime.datetime.now))])
This works great right up until you try running migrations:
- First time it creates a
- Create model Thing
- Then you run the same command again and again, and will always generate a new migration
- Alter field day on thing
class Migration(migrations.Migration): dependencies = [ ('app', '0005_auto_20181015_2203'), ] operations = [ migrations.AlterField( model_name='thing', name='day', field=models.DateTimeField(validators=[django.core.validators.MinValueValidator(datetime.datetime(2018, 10, 15, 22, 3, 41, 390769))]), ), ]
The issue being that the now()
is being evaluated and thus is always different.
I got it 50% working with this diff:
diff --git a/django/db/migrations/serializer.py b/django/db/migrations/serializer.py index 911cf0f..ef2ad43 100644 --- a/django/db/migrations/serializer.py +++ b/django/db/migrations/serializer.py @@ -12,7 +12,7 @@ import uuid from django.db import models from django.db.migrations.operations.base import Operation from django.db.migrations.utils import COMPILED_REGEX_TYPE, RegexObject -from django.utils.functional import LazyObject, Promise +from django.utils.functional import LazyObject, Promise, SimpleLazyObject from django.utils.timezone import utc from django.utils.version import get_docs_version @@ -273,6 +273,8 @@ def serializer_factory(value): from django.db.migrations.writer import SettingsReference if isinstance(value, Promise): value = str(value) + elif isinstance(value, SimpleLazyObject): + value = value._setupfunc elif isinstance(value, LazyObject): # The unwrapped value is returned as the first item of the arguments # tuple.
Turns the migrations into:
class Migration(migrations.Migration): dependencies = [ ('app', '0004_auto_20181015_2203'), ] operations = [ migrations.AlterField( model_name='thing', name='day', field=models.DateTimeField(validators=[django.core.validators.MinValueValidator(datetime.datetime.now)]), ), ]
While it's a great improvement, it still generates a new one every time. I'm a little over my head with this one, this code is very dense. I could keep looking at it tomorrow, but i need someone to point me in the right direction + where in the world do i put the tests for this thing?? Thanks.
Change History (12)
comment:1 by , 6 years ago
Component: | Uncategorized → Migrations |
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Description: | modified (diff) |
Summary: | SimpleLazyObject Causing Migrations to be created over and over → Infinite migrations when using SimpleLazyObject in field arguments |
Triage Stage: | Unreviewed → Accepted |
Type: | Uncategorized → Bug |
comment:2 by , 6 years ago
Yup, that looks correct. Only when we compare its _setupfunc
do we get a hit. Where exactly does that occur? (The comparison)
My local tests seem to work, im going to write a proper test as soon as i have time. https://github.com/django/django/pull/10516
comment:3 by , 6 years ago
@TimGraham Looks like i just hit a snag with the SimpleLazyObject test in django/tests/migrations/test_writer.py:256
now sure how to overcome this, I see the need for this to be evaluated right then and there, and i also see a need for the underlying object to be compared.. Not sure how you want to handle this.
comment:4 by , 6 years ago
Has patch: | set |
---|---|
Patch needs improvement: | set |
As I mentioned on the PR serializing MinValueValidator(SimpleLazyObject(datetime.datetime.now))
to MinValueValidator(datetime.datetime.now)
won't work as it generates broken code
> from datetime import datetime > from django.core.validators import MinValueValidator > MinValueValidator(datetime.now)(datetime(2018, 5, 15)) TypeError: can't compare datetime.datetime to builtin_function_or_method
The only solution I can think of involves adjusting SimpleLazyObject.__eq__(other)
to special case isinstance(other, SimpleLazyObject)
but that would break some assumptions
e.g. if it was to be changed
> datetime.datetime.now() == datetime.datetime.now() False > SimpleLazyObject(datetime.datetime.now) == SimpleLazyObject(datetime.datetime.now) True # Currently False
comment:5 by , 6 years ago
@SimonCharette This somewhat does the trick but still break things...
diff --git a/django/utils/functional.py b/django/utils/functional.py index 1481bf4a5e..4da48bdb1b 100644 --- a/django/utils/functional.py +++ b/django/utils/functional.py @@ -209,6 +209,9 @@ empty = object() def new_method_proxy(func): def inner(self, *args): + if args and isinstance(args[0], type(self)): + return func(self._wrapped, args[0]._wrapped) if self._wrapped is empty: self._setup() return func(self._wrapped, *args)
What do you suggest?
comment:6 by , 6 years ago
The more I think of it the more I feel like the infinite migrations here are expected and this is misuse of SimpleLazyObject
.
SimpleLazyObject(foo)
is really just a way of saying execute foo
on first access but not now as demonstrated by the following snippet
In [1]: from datetime import datetime In [2]: from django.utils.functional import SimpleLazyObject In [3]: lazy_now = SimpleLazyObject(datetime.now) In [4]: lazy_now.strftime("%Y-%m-%d %H:%M:%S") Out[4]: '2018-10-16 15:26:54' In [5]: import time; time.sleep(1) In [6]: lazy_now.strftime("%Y-%m-%d %H:%M:%S") Out[6]: '2018-10-16 15:26:54'
In this sense MinValueValidator(SimpleLazyObject(datetime.now))
is really just a slightly deferred datetime.now()
as the first time the wrapped function is evaluated the returned value is persisted. In other words MinValueValidator(SimpleLazyObject(datetime.now))
is really equivalent to MinValueValidator(datetime.now())
and is the same class of problems the DateTimeField(default=datetime.now())
fields.W161
check is dealing with.
Now what I think you were trying to do here is provide a validator that makes sure it's not possible to provide a past datetime value and this was not working appropriately as the validator was actually performing a check against the first time the SimpleLazyObject
wrapped function was resolved. For this purpose you should be using django.utils.lazy
instead.
In [1]: from datetime import datetime, timedelta In [2]: from django.utils.functional import lazy In [3]: lazy_now = lazy(datetime.now, datetime)() In [4]: from django.core.validators import MinValueValidator In [5]: validator = MinValueValidator(lazy_now) In [6]: lazy_now Out[6]: datetime.datetime(2018, 10, 16, 14, 38, 59, 125760) In [7]: type(lazy_now) Out[7]: django.utils.functional.__proxy__ In [8]: validator(datetime.now()) ValidationError: [u'Ensure this value is greater than or equal to 2018-10-16 14:39:24.060516.'] In [9]: validator(datetime.now() + timedelta(seconds=1)) # account for the function call duration
The only issue here is that Promise
serialization assumes lazy
is always dealing with strings because the main use of them internally is for translations through the gettext
function #21008.
In summary, this is a misuse of SimpleLazyObject
and lazy
should have been used instead but Promise
serialization currently assumes it's only used for string values and it should be adjusted to deal with other types as well. What I suggest doing is inspecting wrapped types of the promise object and only perform a str
on the value if the types are (str,)
. Else the value should be deconstructed as a django.utils.functional.lazy
import and a lazy(...)()
reconstruction.
comment:7 by , 6 years ago
@SimonCharette all this makes 100% sense.
As a followup, going back to my original example, what is the acceptable migration that is to be generated?
class Migration(migrations.Migration): initial = True dependencies = [ ] operations = [ migrations.CreateModel( name='Thing', fields=[ ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')), ('day', models.DateTimeField(validators=[django.core.validators.MinValueValidator(lazy(datetime.datetime.now, datetime.datetime))])), ('day2', models.DateTimeField(validators=[hello.app.models.valid_less_than2])), ], ), ]
or perhaps
('day', models.DateTimeField(validators=[django.core.validators.MinValueValidator(datetime.datetime.now))])),
If we can agree to what is the desired outcome, perhaps i'll be able to work backwards.
comment:8 by , 6 years ago
Well I think the current behaviour of the serializer regarding SimpleLazyObject
is correct but Promise
objects handling would need to be adjusted.
In the end we're facing the same __eq__
proxy issue I mentioned in previous comments; lazy(datetime.datetime.now, datetime.datetime)()
needs to be passed at validator initialization time but lazy(datetime.datetime.now, datetime.datetime)() != lazy(datetime.datetime.now, datetime.datetime)()
because datetime.datetime.now() != datetime.datetime.now()
.
I beginning to think that the only way is to resolve this is allow callables to be passed as BaseValidator(limit_value)
. That would allow you to specify MinValueValidator(datetime.now)
which is easily deconstructible and doesn't involve promise wrapping. Thoughts?
comment:9 by , 6 years ago
So something around the lines of?
diff --git a/django/core/validators.py b/django/core/validators.py index c1c9cd1c87..c2fb66e241 100644 --- a/django/core/validators.py +++ b/django/core/validators.py @@ -317,8 +317,11 @@ class BaseValidator: def __call__(self, value): cleaned = self.clean(value) - params = {'limit_value': self.limit_value, 'show_value': cleaned, 'value': value} - if self.compare(cleaned, self.limit_value): + limit_value = self.limit_value + if callable(limit_value): + limit_value = self.limit_value() + params = {'limit_value': limit_value, 'show_value': cleaned, 'value': value} + if self.compare(cleaned, limit_value): raise ValidationError(self.message, code=self.code, params=params)
comment:10 by , 6 years ago
Resolution: | → wontfix |
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Status: | new → closed |
Looks reasonable to me. Let's open a separate ticket for that so that we have a record of the "wontfix" decision to support SimpleLazyObject
in field arguments.
comment:11 by , 6 years ago
Sounds good Tim.
Javier could you file a new feature request for the callable limit_value
in BaseValidator
? The diff you suggested in comment:9 should do. It would also require tests and docs adjustments but this is something we can discuss on the new ticket/PR. Thanks for continued efforts here by the way!
I think the problem is this:
The two must be considered equal to prevent infinite migrations. See 91f701f4fc324cd2feb7dbf151338a358ca0ea18 for a similar issue.