Adding a bulk_save method to models

[Tom Forbes]

Hello all,

I’d love for some feedback on an idea I’ve been mulling around lately, namely adding a bulk_save method to Dango.

A somewhat common pattern for some applications is to loop over a list of models, set an attribute and call save on them. This unfortunately can issue a lot of database queries which can be a significant slowdown. You can work around this by using ‘.update()’ in some cases, but not all.

It seems it would be possible to use a CASE statement in SQL to handle bulk-updating many rows with differing values. For example:

SomeModel.object.filter(id__in=[1,2]).update(
    some_field=Case(
        When(id=1, then=Value('Field value for ID=1')),
        When(id=2, then=Value('Field value for ID=2'))
    )
)

I’ve made a ticket for this here: https://code.djangoproject.com/ticket/29037

I managed to get a 70x performance increase using this technique on a fairly large table, and it seems it could be applicable to many projects just like bulk_create.

The downsides to this is that it can produce very large SQL statements when updating many rows (I had MySQL complain about a 10MB statement once), but this can be overcome with batching and other optimisations (i.e the same values can use WHEN id IN (x, y, z) rather than 3 individual WHEN statements).

I’m imagining an API very similar to bulk_create, but spend any time on a patch I thought I would ask if anyone have any feedback on this suggestion. Would this be a good addition to Dango?

[Neal Todd]

Hi Tom,

A built-in bulk save that's more flexible than update would certainly be nice. Just in case you haven't come across it though, there is a package called django-bulk-update:

https://github.com/aykut/django-bulk-update

I've found it very useful on a number of occassions where update isn't quite enough but the loop-edit-save pattern is too slow to be convenient.

Probably some useful things in there when considering the API and approach.

Cheers, Neal 

[Tom Forbes]

Hey Neal,

Thank you very much for pointing that out, I actually found out about this package as I was researching the ticket - I wish I had known about this a couple of years ago as it would have saved me a fair bit of CPU and brain time!

I think that module is a good starting point and proves that it’s possible, however I think the implementation can be improved upon if we bring it inside core. I worked on a small PR to add this and the implementation was refreshingly simple. It still needs docs, a couple more tests and to fix a strange error with sqlite on Windows, but overall it seems like a lot of gain for a small amount of code.

Tom

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[Neal Todd]

Hi Tom,

That's great, should be a helpful addition to core. Will follow the ticket and PR.

Neal

(Apologies - I hadn't spotted that you'd already referenced django-bulk-update in your ticket when I left my drive-by comment!)

[Tim Graham]

I wanted to ask about naming of the new method. Currently the proposed name is "QuerySet.bulk_save()" but I think it's a bit confusing since it uses QuerySet.update(), not Model.save(). It works similarly to QuerySet.bulk_update() from https://github.com/aykut/django-bulk-update but the arguments are a bit different.

Josh's comment on the PR: "Since this only works for instances with an pk, do you think that bulk_update would be a better name? The regular save() method can either create or update depending on pk status which may confuse users here."

And Tom's reply: "I considered this, but queryset.update() is the best 'bulk update' method. I didn't want to confuse the two, this is more about saving multiple model fields with multiple differing values, gene bulk_save. Open to changing it though."

[Raphael Michel]

Hi,

I'd be very careful about calling it bulk_save(), since calling
it something with save() very strongly suggests that it calls pre_save
or post_save signals.

Best
Raphael


Am Fri, 14 Sep 2018 07:56:38 -0700 (PDT)
schrieb Tim Graham <timog...@gmail.com>:

> >> <https://github.com/django/django/pull/9606/files#diff-5b0dda5eb9a242c15879dc9cd2121379R473>

> >> <https://groups.google.com/d/msgid/django-developers/5988d579-7843-4c42-a6f9-1e389c58ece6%40googlegroups.com?utm_medium=email&utm_source=footer> .

[Tom Forbes]

My original reasoning was that Queryset.update() already bulk updates rows, so the bulk prefix seems a bit redundant here (how do you bulk something that already does something in bulk?). .save() however operates on a single object, so the bulk prefix seems more appropriate and easier to understand.

I agree bulk_save() maybe is not the best name as people might expect signals to be sent, but are there any suggestions other than bulk_update()? Maybe something more accurate, like bulk_update_fields()? Or bulk_save_fields()?

To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/20180914173155.41685505%40kvothe.

[Adam Johnson]

Bikeshed time.

I'm also against bulk_save for the same reason that it implies save().

bulk_update sounds okay to me, update() is indeed already a 'bulk' operation but it could be claimed this is doing a 'bulk' amount of update operations.

bulk_update_fields also sounds good, the longer method name is probably balanced by the lower frequency of use.

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--

Adam

[charettes]

I also dislike bulk_save() for the same reasons.

I feel like bulk_update makes the most of sense given it has a signature similar to bulk_create where an iterable of model instances must be passed we're really just performing an update.

To the bulk_update and update is the natural analogous to what bulk_create is to create; bulk_update_fields feels too verbose and breaks the symmetry of bulk_create/create for update.

Cheers,

Simon

[Tobias Kunze]

On 18-09-15 23:15:10, Adam Johnson wrote:
>> I agree bulk_save() maybe is not the best name as people might expect
>> signals to be sent, but are there any suggestions other than bulk_update()?
>> Maybe something more accurate, like bulk_update_fields()? Or
>> bulk_save_fields()?
>

>bulk_update_fields also sounds good, the longer method name is probably
>balanced by the lower frequency of use.

bulk_update_fields() sounds fine to me, as it makes clearer what
happens. With bulk_update() alone, I'd expect the exact analogous
action to update() to occur, since we're already used to that pattern
from create() vs bulk_create().

update_fields() alone may also work. Upside: it's shorter. Downside:
it's not immediately clear that it takes an iterable and not an
instance. I'd be happy with both options.

Best regards,
Tobias

[Tom Forbes]

Thank you all for the feedback, I’ve changed the method to be bulk_update() as this seems to be the most liked option. Naming things is hard, and while bulk_update() isn’t perfect I think it’s a bit better than bulk_update_fields() or just update_fields().

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