Migration to create TextField with db_index=True crashes on MySQL

[Daniel Quinn]

I have a GPL project that uses Django as its base that typically works using SQLite. However, some of my users have been trying to use it with MySQL and this has presented a strange annoyance.

In one of my models, I have this line:

content = models.TextField(db_index=True)

this generates a migration that looks like this:

...
('content', models.TextField(db_index=True)),
...

...which seems reasonable enough to me. However, while this works just fine in SQLite and PostgreSQL, users attempting to install the project using MySQL were treated to this explosion when they ran manage.py migrate:

django.db.utils.OperationalError: (1170, "BLOB/TEXT column 'content' used in key specification without a key length")

Now understandably, if you're writing a project *for* MySQL, you wouldn't put an index on a TextField because you know that MySQL doesn't support that, but in the interests of portability, I would think a warning, coupled with ignoring the index specification would be more appropriate.

For now, I've just had to rewrite the migration to look like this:

('content', models.TextField(db_index=("mysql" not in settings.DATABASES["default"]["ENGINE"]))),

I'm not proud of that one, but it works for now :-)

There's a longer discussion about this issue on my project's GitHub issue page here: ​https://github.com/danielquinn/paperless/issues/183 if you're interested.

​https://github.com/danielquinn/paperless/issues/183#issuecomment-280863310

[Simon Charette]

Related to #2495.

I also think the correct way of handling that would be to make MySQL's schema editor ignores db_index on TextField.

[Zubair Alam]

Please find the patch at pull request https://github.com/django/django/pull/8118

[Mariusz Felisiak]

​PR

[GitHub <noreply@…>]

In 5cff2cb4:

Refs #27859 -- Refactored BaseDatabaseValidation to use check_field_type().

Thanks Tim Graham for the review.

[Tim Graham <timograham@…>]

In b3eb6eaf:

Refs #27859 -- Added DatabaseWrapper.display_name.

Thanks Tim Graham for the review.

[GitHub <noreply@…>]

In 538bf434:

Fixed #27859 -- Ignored db_index for TextField/BinaryField on Oracle and MySQL.

Thanks Zubair Alam for the initial patch and Tim Graham for the review.

[Claude Paroz]

I have a ​PR that partially reverts this fix and allows for TextField.db_index for MySQL. Thoughts?