#32827: Squashing migrations isn't very effective in complicated projects;
rewrite
docs to provide a preferred manual trimming process
-------------------------------------+-------------------------------------
Reporter: Mike Lissner | Owner: Mike
Type: | Lissner
Cleanup/optimization | Status: assigned
Component: Documentation | Version: dev
Severity: Normal | Resolution:
Keywords: migrations, | Triage Stage: Accepted
squashmigration, documentation |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Natalia Bidart):
Replying to [comment:4 David Sanders]:
> @Carlton & Nessita:
Hello! Thanks for including me.
> I agree there needs to be a bit in the docs for this; Mike has stepped
down as he doesn't have any time but I have some time:
>
> - I agree that it should be "manually squashing" vs "automatic
squashing"
I don't object to this though I don't understand exactly which scenario
would be "automatic squashing": even when running `manage.py
squashmigrations arg1 arg2`, the user has to perform manual steps
afterwards (unless I'm missing something, which can be the case!)
What I mean is that, in my experience, squashing migrations is always a
manual operation (beyond running the management command, that is). There
is more or less steps, and one may or may not use the management command.
So if you ask me I would organize the docs roughly like this:
1. Squashing migrations
You can use the provided management command ... <details here for this
option>.
If the above does not work or does not suit your needs for whatever reason
(<examples>), you could instead:
<details for option 2>
<details for option 3>
I guess my point is that at most I would use labels to distinguish the
options like: using the mgmt command, resetting, replacing.
> - I've only tried `squashmigrations` twice but both times failed and
ended up costing me quite a few hours that my boss wasn't too happy about
:D
I've did it multiple times are previous work but it required a non trivial
amount of coordination involving migration landing freezes, deployment,
squashing, migration unfreeze. We only did this when migrate was too slow
when running tests.
> - The first time there was this circular dependency nightmare that I
couldn't escape
Luckily we did not have this! But we did have a test check to ensure apps
would not create circular dependencies, so I'm sure that helped.
> - The second I had to write some custom serialisation to deal with
django-pgtrigger & in the end it didn't work out (I don't remember all the
details as this was a few months ago)
> - I decided to finally manually squash them, but using the "replaces"
feature that `squashmigrations` uses – and it worked a treat plus it
didn't take very long!
> - The key thing to remember is that you *must* note down all the non-
elidable operations.
:see_no_evil:
> - These are the steps I used (they'll need to be tailored for general
public):
> - I rm'd all my migrations
> - ran makemigrations (giving it a distinct new initial name)
> - used git to restore all the rm'd migrations
> - copy all the filenames into the `replaces` attribute
> - copied all my non-elidable operations *plus any dependencies* into
the new initial migration
> - wait until all deployments have been migrated out of the "replaces
zone" then follow the rest of the instructions on the docs
I never tried this procedure!
> Sounded like Jessamyn had some issues with missing the custom user model
- I didn't encounter this issue even though my user model was in the app I
squashed 🤔
>
> I'd like to forward a PR if that's ok.
Yes please!
--
Ticket URL: <https://code.djangoproject.com/ticket/32827#comment:5>
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