Cannot use migrations if model field specifies validators

[Tim Graham]

My assumption is that we shouldn't try to serialize the validators attribute.

Traceback:

  File "/home/tim/.virtualenvs/foo/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 185, in serialize
    raise ValueError("Cannot serialize: %r" % value)
ValueError: Cannot serialize: <django.core.validators.RegexValidator object at 0x1bacc10>

[Tim Graham]

After discussion with loic84 in #django-developers, it appears that validators should be serializable and that this error is due to the regex used by the username field of contrib.auth.models.AbstractUser not being serializable. Loic says validators may be useful for data migrations, hence a possible reason not to exclude them entirely.

[loic84]

POC ​https://github.com/loic/django/compare/ticket21275.

[loic84]

Another POC around the concept of a deconstructible decorator.

​https://github.com/loic/django/tree/ticket21275_take2.

[loic84]

PR ​https://github.com/django/django/pull/1783.

Note that this will be needed if/when #21295 is merged.

[Tim Graham <timograham@…>]

In e565e1332ddfbb44fe7e6139375e3c243af7398d:

Fixed #21275 -- Fixed a serializer error when generating migrations for contrib.auth.

The migration serializer now looks for a deconstruct method on any object.

[Sasha Romijn]

Shouldn't this also be added to the release notes? If anyone has their own model with a custom RegexValidator where they currently pass a compiled regex, they'll need to pass a string now, or they will also hit this issue, right?

[Daniel Hahler]

Please note that there is #22255, which is about adding a new flags kwarg.

[Sasha Romijn]

I've included this in the release notes with ​https://github.com/django/django/commit/4d0c5f61427a8e67552ee2d777fffbadc7aff3b2

[Mariusz Felisiak <felisiak.mariusz@…>]

In 194ca770:

Refs #21275 -- Added more tests for @deconstructible decorator.