I want to qualify this by saying that personally I've always thought of
(and likely will continue to think of) model objects as a snapshot of
what's in the database, and use the more explicit queryset.update() when
that snapshot is NOT what I want to save back to the database. So I may be
somewhat biased against this idea to start with. :)
That said, as Simon mentioned, don't we already effectively support this?
>>> from django.db import connection
>>> from food.models import Food
>>> f = Food.objects.only('pk').first()
>>> f.name = 'foo'
>>> f.save()
>>> connection.queries
[{'sql': 'SELECT "food_food"."id" FROM "food_food" ORDER BY
"food_food"."food_code" ASC LIMIT 1', 'time': '0.003'}, {'sql': 'UPDATE
"food_food" SET "name" = \'foo\' WHERE "food_food"."id" = 5542', 'time':
'0.010'}]
(For the record, this model has many more fields.)
Of course, if you need other fields to calculate whatever needs to be saved
back to the DB, you should add them to .only(). But in any case it protects
you from silently losing data by forgetting to include them in
update_fields (at the risk of generating extra queries if you forget to
include them in .only(), but at least you can see those and optimize them
away).
Consider me relatively ambivalent about the idea...perhaps +0 on a Meta- or
save()-level implementation and -0 on a settings-based approach (given the
red flags already raised).
Cheers,
*Tobias McNulty*Chief Executive Officer
tob...@caktusgroup.com
www.caktusgroup.com
On Thu, Mar 14, 2019 at 11:20 AM charettes <charett...@gmail.com> wrote:
> +1 to what Adam said. Turning this feature on by default would be
> certainly cause a lot of bugs in application
> expecting a "coherent state" to be persisted on .save().
>
> That would effectively make most of the Form.clean(), Model.clean() and
> DRF serializer logic in charge of
> validating a field value based on another field value useless as another
> process could race to update the
> row to another state unless you've defined database level constraints as
> well.
>
> > ... performance *benefits* of executing smaller UPDATE statements that
> write fewer columns.
>
> FWIW the current .save() logic ignores deferred fields so it's currently
> possible to work around the
> rare cases where updating a large column causes a slow UPDATE query.
>
> > I'm not sure if a global setting is a good idea. Code in third-party
> apps would need to work with both configurations?
>
> Agreed, it feels like a Meta or .save() option would be more appropriate,
> we already have a select_on_save
> option for example which is even more specific than this use case.
>
> At this point I'm not convinced that it's something Django should
> implement internally as it is possible to implement
> an abstract model in charge of tracking "dirty" fields and overriding
> `.save` to make `update_fields` default to it
> as demonstrated by the few existing third-party solutions.
>
> Simon
>
> Le jeudi 14 mars 2019 09:49:59 UTC-4, Tim Graham a écrit :
>>
>> I'm not sure if a global setting is a good idea. Code in third-party apps
>> would need to work with both configurations?
>>
>> On Thursday, March 14, 2019 at 8:39:05 AM UTC-4, Daniel Tao wrote:
>>>
>>> I agree with this:
>>>
>>> > Therefore it seems like it would be a breaking change which is hard to
>>> communicate to users, a complicated situation.
>>>
>>> This is why I'm recommending that this behavior be opt-in via a Django
>>> setting such as SAVE_UPDATE_DIRTY_FIELDS_ONLY. Application developers
>>> would then need to make a determination for themselves whether or not to
>>> enable it.
>>>
>>> Adam is right to point out an edge case where this behavior could lead
>>> to subtle bugs. Not surprisingly, it's related to concurrency (which is
>>> where subtle bugs tend to live)! In response, I would point out that the
>>> whole point of this proposal is to address a *different* edge case,
>>> also related to concurrency, which I personally believe is more common,
>>> though it's admittedly hard to know without a lot of empirical data.
>>> (Actually it's also present in the scenario Adam describes, and he
>>> addresses this: "Sure there's a race condition and the results of either
>>> process 1 or 2 can be lost.")
>>>
>>> From the perspective of an application developer, if this functionality
>>> were available to me, I'd want to look at the code and ask myself this
>>> question: which is more work?
>>>
>>> - Are there lots of places where the code is setting a small number
>>> of attributes and calling save() without update_fields, where concurrent
>>> requests (or background jobs) could override each other's writes and user
>>> data could be lost? Should I leave SAVE_UPDATE_DIRTY_FIELDS_ONLY
>>> *disabled*, enumerate these code paths and add update_fields to all
>>> of them?
>>> - Are there lots of places where the code is setting model
>>> attributes whose values are derived from a combination of other
>>> attributes,
>>> not all of which are set? Should I *enable*
>>> SAVE_UPDATE_DIRTY_FIELDS_ONLY, enumerate these code paths and wrap them
>>> in
>>> select_for_update to ensure the data doesn't get into an inconsistent
>>> state?
>>>
>>>
>>> On Monday, January 28, 2019 at 8:00:21 AM UTC-6, Daniel Tao wrote:
>>>>
>>>> Hi!
>>>>
>>>> This is my first post on this list. I recently left a comment on #27017
>>>> <https://code.djangoproject.com/ticket/27017#comment:3> requesting to
>>>> re-open the topic of only saving dirty fields in save(). Tim Graham
>>>> helpfully directed to #4102
>>>> <https://code.djangoproject.com/ticket/4102> and advised that I make
>>>> the proposal on the dev mailing list, so that's what I'm doing :)
>>>>
>>>> I've gone through the history of #4102 and taken notes on the
>>>> challenges that arose when this was first attempted 12 years ago (!). The
>>>> TL;DR is that, while there were indeed quite a few complications, I don't
>>>> see anything that came up that should be considered a flat-out showstopper.
>>>> Rather, what happened was that at about the 69th comment
>>>> <https://code.djangoproject.com/ticket/4102#comment:69>, back in 2012,
>>>> the conversation shifted as Matt Long made this observation:
>>>>
>>>> what started as a simple opt-in feature request of adding a field
>>>>> white-list to the Model's save function morphed into a complicated dirty
>>>>> flag approach that obviously has many edge cases and performance
>>>>> implications given that this ticket has been open for 5 years now
>>>>
>>>>
>>>> From here it seems things progressed towards the current solution of
>>>> supporting the update_fields argument, and that's where things ended.
>>>> I would like to point out that Matt did *not* advocate for completely
>>>> abandoning all efforts to support dirty field tracking; to the contrary, in
>>>> the same comment he said this (emphasis mine):
>>>>
>>>> Clearly some people feel differently and favor the dirty flag approach
>>>>> for a more hands-off approach. As such, I propose adding support for *both
>>>>> methods*
>>>>
>>>>
>>>> With that in mind, I believe it's worth re-opening this discussion. For
>>>> a fairly lengthy justification, see my aforementioned comment on #27017
>>>> <https://code.djangoproject.com/ticket/27017#comment:3>. I'll copy the
>>>> effective TL;DR of the proposal here for convenience:
>>>>
>>>> In my opinion Django could make most code bases inherently more
>>>>> resilient against latent race conditions by implementing some form of
>>>>> dirty
>>>>> field tracking and effectively providing the functionality of
>>>>> update_fields automatically. I would like to propose a new setting,
>>>>> something like SAVE_UPDATE_DIRTY_FIELDS_ONLY, to change the ORM's
>>>>> default behavior so that calls to Model.save() only update the fields
>>>>> that have been set on the model instance. Naturally for backwards
>>>>> compatibility this setting would be False by default.
>>>>
>>>>
>>>> As for the concerns that were raised when this was first attempted, I
>>>> will now attempt to summarize what I found along with, in most cases, a bit
>>>> of editorializing from me.
>>>>
>>>> Performance
>>>>
>>>> The performance angle was first explored in a comment
>>>> <https://code.djangoproject.com/ticket/4102#comment:7> that said it
>>>> "doesn't look good" and provided some benchmarks showing a performance hit
>>>> from 0.17s to 2.64s for setting an attribute using the timeit
>>>> <https://docs.python.org/3/library/timeit.html> package. I didn't see
>>>> anyone point out that the timeit method defaults to executing code *a
>>>> million times*; so the throughput of the operation went from about 6
>>>> million to closer to 400 thousand times per second. (The percentage change
>>>> is indeed significant, but this doesn't *smell* like a potential
>>>> bottleneck.)
>>>>
>>>> It was noted in a couple
>>>> <https://code.djangoproject.com/ticket/4102#comment:14> places
>>>> <https://code.djangoproject.com/ticket/4102#comment:29> that it seems
>>>> potentially shortsighted to focus so much on the performance of getting and
>>>> setting attributes without taking into account the potential performance
>>>> *benefits* of executing smaller UPDATE statements that write fewer
>>>> columns. As far as I can tell, no one in the thread on #4102 actively
>>>> investigated the latter.
>>>>
>>>> Based on the unlikelihood of attribute setting representing a
>>>> performance bottleneck, and the lack of data for the performance impact of
>>>> executing smaller updates, I would consider the performance discussion
>>>> largely a distraction. Though I do think it's worth measuring the latter.
>>>>
>>>> Compatibility
>>>>
>>>> It was observed that the two approaches considered on the ticket
>>>> (overriding __setattr__ or defining custom property setters) would not
>>>> work with obj.__dict__.update
>>>> <https://code.djangoproject.com/ticket/4102#comment:12>, which is
>>>> apparently an optimization you can find prescribed on some blogs out int he
>>>> wild. This supports the premise that this behavior should be opt in, so
>>>> devs who are using incompatible techniques can stay away (unless/until
>>>> they've removed those optimizations from their code).
>>>>
>>>> I had already suggested putting this functionality behind a setting, so
>>>> I think we're good here.
>>>>
>>>> Correctness
>>>>
>>>> There were some bugs related to
>>>> <https://code.djangoproject.com/ticket/4102#comment:39> multi-table
>>>> inheritance <https://code.djangoproject.com/ticket/4102#comment:67>.
>>>> It appears these were both followed by patches with proposed fixes.
>>>>
>>>> Another bug was that fields updated in pre_save such as
>>>> DateTimeField(auto_now=True) were not being updated
>>>> <https://code.djangoproject.com/ticket/4102#comment:60>. There was a
>>>> patch to fix this as well.
>>>>
>>>> The point was raised that this approach poses a problem for mutable
>>>> field types <https://code.djangoproject.com/ticket/4102#comment:68> (e.g.,
>>>> ArrayField). Presumably one approach for addressing that would be the
>>>> one employed by django-save-the-change here
>>>> <https://github.com/karanlyons/django-save-the-change/blob/master/save_the_change/decorators.py#L121>
>>>> .
>>>>
>>>> One gets the sense from following the thread that, surely, there would
>>>> be more bugs discovered were someone to resurrect this work. However,
>>>> again, I don't see any showstoppers here. We're talking about an advanced
>>>> framework with a large set of features, and so naturally it's going to be
>>>> difficult to get this right while taking all supported use cases into
>>>> account.
>>>>
>>>> Conclusion
>>>>
>>>> As I said up top, I think it's worth re-opening #27017, or creating a
>>>> new ticket for the behavior it was requesting (updating only dirty fields
>>>> automatically in save()) where future contributors might discuss
>>>> possible approaches and, with any luck, submit pull requests. I believe it
>>>> would be both possible and helpful, and I also *suspect* that the
>>>> process would go a bit more smoothly today, as the project has a huge suite
>>>> of tests to catch possible edge cases and provide greater confidence about
>>>> the correctness of any patches submitted.
>>>>
>>>> P.S. I'm a little worried that someone will now direct me to *another*
>>>> long thread I should read filled with even more reasons this idea caused
>>>> problems in the past. If so, no worries! I'll do it. I've already come this
>>>> far :)
>>>>
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