#30591: bug in manage.py migrate when changing the field type of a foreign key
with
a constraint
--------------------------------------------+------------------------
Reporter: Cornelis Poppema | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 2.2
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
--------------------------------------------+------------------------
I have two models and a ForeignKey. In this project I'm still using django
1.11 (still busy migrating to python3) but I have reproduced this in
django 2.2.2
Models are Call and Client. Call has a ForeignKey to a
PositiveIntegerField on Client. I'm changing the type of this field to a
CharField, but django/mysql cannot drop or add constraints.
I found this issue: https://code.djangoproject.com/ticket/28305. I had
hoped I was still using a version of 1.11 that didn't include the backport
but unfortunately for me I did.
Before creating this ticket I also came across
https://code.djangoproject.com/ticket/30152 which is also related to my
initial findings. As
https://code.djangoproject.com/ticket/30152#comment:10 says, the internals
are checking for relations that might have constraints, but in 30152's and
my case it doesn't find the relation. As a result the foreign key
constraint isn't dropped resulting in the error:
{{{
django.db.utils.OperationalError: (1833, "Cannot change column 'code':
used in a foreign key constraint
'myapp_call_clientcode_907d4acf_fk_myapp_client_code' of table
'test_test.myapp_call'")
}}}
After either applying the patch
https://code.djangoproject.com/attachment/ticket/30152/testcase_and_rough_fix.patch
or simply (temporarily) removing related_name='+' I get past this
OperationError. Instead I am confronted with:
{{{
django.db.utils.IntegrityError: (1215, 'Cannot add foreign key
constraint')
}}}
From what I've read about MySQL constraints this is caused by the
different field types. Changing the type of the original field doesn't
change the type of the foreign key field and the constraint cannot be re-
added after dropping it.
I created a project on github: https://github.com/cpoppema/django-migrate-
constraint-bug, ignore the master-branch for now.
Branch with original issue: https://github.com/cpoppema/django-migrate-
constraint-bug/tree/cannot-change-column-used-in-a-foreign-key-constraint
Branch with issue without related_name='+': https://github.com/cpoppema
/django-migrate-constraint-bug/tree/cannot-add-foreign-key-constraint
Known workaround that I've come up with is:
run a migration to change ForeignKey on Call to PositiveIntegerField
run a migration to change the PositiveIntegerField on Client to CharField
run a migration to change the PositiveIntegerField on Call to ForeignKey
With this, both fields end up as a CharField. I'd rather have Django
generate either multiple migrations or do an ALTER TABLE for related
fields.
--
Ticket URL: <https://code.djangoproject.com/ticket/30591>
Django <https://code.djangoproject.com/>
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