#32689: Infinite AlterField Migrations due to default callable object missmatch
-------------------------------------+-------------------------------------
Reporter: Samuel Bishop | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 3.2
Severity: Normal | Resolution:
Keywords: infinite, | Triage Stage:
makemigrations, migrations, | Unreviewed
callable, default, field |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Description changed by Samuel Bishop:
Old description:
> `makemigration` will generate infinite migrations if the callable path is
> miss-matched such that you end up with two discreet callable objects (I'm
> still trying to identify where exactly the migration framework is
> generating this missmatch, I can see the difference stepping through
> `MigrationAutodetector.generate_altered_fields` in the debugger but I'm
> not sure if the root cause is inside `generate_altered_fields`, another
> method on `MigrationAutodetector` or somewhere else entirely...) then the
> same migration is generated every time you run the command which results
> in no actual change to the database.
>
> In my case it's from a custom field library being used as a primary key.
>
> I was importing `from ulid.api import new as new_ulid` and setting
> `default=new_ulid`, but unless I swap to `import ulid.api.api` and set
> `default=ulid.api.api.Api.new` I just get a new identical migration
> altering the field's default value to `ulid.api.api.Api.new` every time i
> run `makemigrations`
>
> I would have expected either a clear warning detailing why my choice of
> callable object for the `default=` will cause issues, or I would have
> expected `makemigrations` to not mutate my callable like this.
New description:
`makemigration` will generate infinite migrations if the callable path is
miss-matched such that you end up with two discreet callable objects (I'm
still trying to identify where exactly the migration framework is
generating this missmatch, I can see the difference stepping through
`MigrationAutodetector.generate_altered_fields` in the debugger but I'm
not sure if the root cause is inside `generate_altered_fields`, another
method on `MigrationAutodetector` or somewhere else entirely...) then the
same migration is generated every time you run the command which results
in no actual change to the database.
In my case it's from a custom field library being used as a primary key.
I was importing `from ulid.api import new as new_ulid` and setting
`default=new_ulid`, but unless I swap to `import ulid.api.api` and set
`default=ulid.api.api.Api.new` I just get a new identical migration
altering the field's default value to `ulid.api.api.Api.new` every time i
run `makemigrations`
I would have expected either a clear warning detailing why my choice of
callable object for the `default=` will cause issues, or I would have
expected `makemigrations` to not mutate my callable like this.
Root cause: Turns out this is a serialisation fault. The migration
serialiser doesn't throw any kind of error or warning and just mutates the
callable instead of preventing me from using an invalid default callable
that the migration framework wont be able to serialise. A simple wrapper
function added to the custom field library solves the matter entirely,
however the underlying issue of the migration framework behaviour is worth
looking at improving.
--
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Ticket URL: <https://code.djangoproject.com/ticket/32689#comment:2>
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