#24163: Migrations fail when changing from OneToOneField to ForeignKey
----------------------------+--------------------------------------------
Reporter: harnash | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7
Severity: Normal | Keywords: OneToOneField ForeignKey MySQL
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Hi!
My team and I have encountered a problem in migrations when changing
relationship between models from OneToOneField to ForeignKey.
Original thread on django-users can be found here:
https://groups.google.com/forum/?#!topic/django-users/-NUbqzS4jQs
== Environment ==
* OS: Unix (Ubuntu 14.04)/OSX
* Django: 1.7.3
* DB: MySQL 5.6.22
== Steps to reproduce ==
1. Create two models:
{{{
class Model_A(models.Model):
id_A = models.AutoField(primary_key=True)
class Model_B(models.Model):
model_a = models.OneToOneField(Model_A)
}}}
2. Make migrations and perform migration. So far everything is OK.
3. Now change OneToOneField to ForeignKey:
{{{
class Model_A(models.Model):
id_A = models.AutoField(primary_key=True)
class Model_B(models.Model):
model_a = models.ForeignKey(Model_A)
}}}
4. Run migrations (makemigrations) and apply changes to db (migrate). We
get this error:
we get an error:
{{{
Cannot drop index 'model_a_id': needed in a foreign key constraint
}}}
It seems that Django tries to run SQL commands in following order:
{{{
ALTER TABLE 'xxx' DROP INDEX 'yyy' and
ALTER TABLE `xxx` DROP FOREIGN KEY `xxx_yyy_id_{some_hash}_fk_yyy_id`;
}}}
but if above commands are run in reverse order:
{{{
ALTER TABLE `xxx` DROP FOREIGN KEY `xxx_yyy_id_{some_hash}_fk_yyy_id`;
ALTER TABLE 'xxx' DROP INDEX 'yyy' and
}}}
everything is OK.
I have created sample project where it can be reproduced (last three
commits are reproducing above steps): https://github.com/harnash/django-
migration-bug/commits/master
--
Ticket URL: <https://code.djangoproject.com/ticket/24163>
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