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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