While I'm not sure I have any particular suggestion for this, the problem is important enough for a few of the projects I'm maintaining. I have implemented more than a few custom save() methods, post_init/pre_save/post_save signals and complicated ways of setting update_fields and acting on their values and I would really like if there was a simple standard way of tracking dirty fields and saving only them in a reliable way. I'll be grateful if someone finally makes it happen.
Ivan. On Tue, Jan 29, 2019 at 4:43 PM Patryk Zawadzki <pat...@room-303.com> wrote: > I feel conflicted here as I've spent numerous hours under different > contracts fixing bugs caused by missing update_fields and leading to data > loss or inconsistent database states ;) > > On a more serious note, please make this happen. > > With concurrent updates calling save() without update_fields is asking for > trouble, one of our customers has save() methods specifically patched to > raise exceptions under these circumstances because of how easy it is to > mess things up and how hard it is to track them down. > > pon., 28 sty 2019 o 15:00 Daniel Tao <daniel....@gmail.com> napisał(a): > >> 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 :) >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Django developers (Contributions to Django itself)" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to django-developers+unsubscr...@googlegroups.com. >> To post to this group, send email to django-developers@googlegroups.com. >> Visit this group at https://groups.google.com/group/django-developers. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/django-developers/f13f860e-8dc9-4a5d-80c5-f1201e741a9d%40googlegroups.com >> <https://groups.google.com/d/msgid/django-developers/f13f860e-8dc9-4a5d-80c5-f1201e741a9d%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> For more options, visit https://groups.google.com/d/optout. >> > > > -- > Patryk Zawadzki > I solve problems. > > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to django-developers+unsubscr...@googlegroups.com. > To post to this group, send email to django-developers@googlegroups.com. > Visit this group at https://groups.google.com/group/django-developers. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/CANw2pUGuMXs1RmqAHZjaoHuTpNKzVnWRi68bioZegJxkv_QL9Q%40mail.gmail.com > <https://groups.google.com/d/msgid/django-developers/CANw2pUGuMXs1RmqAHZjaoHuTpNKzVnWRi68bioZegJxkv_QL9Q%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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