JSONField default passed to SQL create column query

[ConorMcGee]

Summary: Migrations created to add a JSONField include default, causing query to fail.

Django Version: Django 1.9.2-8

Database and version used: MySQL 5.7.11

Version: Django-MySQL 1.0.1

Hi,
I've been having some inconsistent behaviour with this so sorry if this is confusing. In the current situation I'm stuck in, I have model that was created with a JSONField without any problems, and I'm trying to add another JSONField. It keeps failing because the migrations tries to set a default on the field, which MySQL doesn't allow:
(django.db.utils.OperationalError: (1101, "BLOB, TEXT, GEOMETRY or JSON column 'tables' can't have a default value"))
I think I know why this is happening: https://github.com/django/django/blob/6bf7964023487f2a352084e74aca27aecb354d6c/django/db/backends/mysql/schema.py#L28
That method tells migrations to leave out the default for certain fields, and I believe JSONField would now need to be included among those, as the django_mysql JSONField implicitly sets dict as its default.

I can't immediately see how this could be rectified here. Do you think it requires a bug report for Django? Or maybe I've overlooked something.

I know I've left out full tracebacks and stuff but that's just because I'm pretty sure I've located the issue above. Let me know if you'd like more.

[adamchainz]

Yes that would need fixing at the Django level. You should file a ticket there and link here.

As a workaround you should be able to remove the migration operations on the existing field if you remove the migration operations on it from the auto-generated migration (why is it detected if changed if it's a separate field you're adding❓ ).

[adamchainz]

Also send me the django ticket link, I might be able to help on it :)

[ConorMcGee]

Sorry to be clear it's an AddField migration that's failing. I actually got around it for now by monkey-patching the method in the migration. I'll check back in when I get a chance to file the bug. Thanks!

[adamchainz]

It's the AddField on the new field that fails? 😢

[ConorMcGee]

Yes AddField in this instance. Ticket created here.

[adamchainz]

Ok I've made a PR against Django at django/django#6997, closing this since that will resolve it. If you need it before Django < 1.11 it's possible to fix it with your own database backend by subclassing the existing one, its SchemaEditor class, and including the fixed skip_default method. I can help you with this if you need it!

[ConorMcGee]

Great! Thanks very much. I've already overridden the method for now, as you said.

[geeknam]

@adamchainz just wondering if there's an easy way to monkey patch skip_default? If not, would you be able to provide a snippet on how you subclass the SchemaEditor?

[adamchainz]

Yes you could monkey patch, or better try patchy.patch and create a patch file from the difference between the two: https://github.com/adamchainz/patchy

IT's more than a snippet to subclass the database backend since you need to copy a few files. Here's an example of another one I have done: https://github.com/YPlan/django-perf-rec/tree/master/tests/django18_sqlite3_backend

HTH

[iosifnicolae2]

You can set Default value to None:

[adamchainz]

@iosifnicolae2 or upgrade your Django :)