That's a +1 from me. I've certainly hit the bugs you've mentioned before, 
and I can't think of a good reason not to do dirty field tracking, 
especially if it were to be opt in.

Bikeshedding a little bit, rather than having a global setting, I'd rather 
see an option on Options/Meta so that opt in is per-model. On a related 
note, I'd like to see some real world testing of any solution with packages 
that implement polymorphic models.

On Tuesday, 29 January 2019 01:00:21 UTC+11, 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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