#25253: Migrations do unecessary work when adding `blank=True` to M2M field on
MySQL
----------------------------+----------------------------------
Reporter: trecouvr | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.8
Severity: Normal | Keywords: migrations m2m mysql
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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When running a MySQL migration to add `blank=True` on a `ManyToManyField`,
Django drops the FK constraints on the join table and creates them back.
Here is the changeset for models.py:
{{{
from django.contrib.auth.models import User
from django.db import models
class Banana(models.Model):
- users = models.ManyToManyField(User)
+ users = models.ManyToManyField(User, blank=True)
}}}
And here the corresponding migration:
{{{
..
operations = [
migrations.AlterField(
model_name='banana',
name='users',
field=models.ManyToManyField(to=settings.AUTH_USER_MODEL,
blank=True),
),
]
..
}}}
The resulting sql for MySQL backend is:
{{{
ALTER TABLE `myproject_banana_users` DROP FOREIGN KEY
`myproject_banana_users_user_id_93c9ecfd1bc0dc2_fk_auth_user_id`;
ALTER TABLE `myproject_banana_users` ADD CONSTRAINT
`myproject_banana_users_user_id_60a2450416dd445f_fk_auth_user_id` FOREIGN
KEY (`user_id`) REFERENCES `auth_user` (`id`);
ALTER TABLE `myproject_banana_users` DROP FOREIGN KEY
`myproject_bana_banana_id_4be506ef34515098_fk_myproject_banana_id`;
ALTER TABLE `myproject_banana_users` ADD CONSTRAINT
`myproject_bana_banana_id_43426c15f7d142f5_fk_myproject_banana_id` FOREIGN
KEY (`banana_id`) REFERENCES `myproject_banana` (`id`);
}}}
I checked on PostgreSQL the resulting sql is empty, which looks good to me
since there is no change to issue to the database.
--
Ticket URL: <https://code.djangoproject.com/ticket/25253>
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