#27558: Setting db_index=False on existing ForeignKey causes constraint to be
recreated
-------------------------------------+-------------------------------------
Reporter: Ed Morley | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: 1.10
layer (models, ORM) |
Severity: Normal | Keywords: db-indexes, mysql
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Using:
* Django 1.10.3
* MySQL 5.6
STR:
1) Start with this models.py:
{{{#!python
class Musician(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
class Album(models.Model):
artist = models.ForeignKey(Musician)
name = models.CharField(max_length=100)
class Meta:
unique_together = ('artist', 'name')
}}}
2) Run `./manage.py makemigrations && ./manage.py migrate` to generate and
apply the initial migration.
3) Add `db_index=False` to the `ForeignKey` (to work around the duplicate
index issue discussed [https://groups.google.com/forum/#!topic/django-
developers/3ywugkcaxqs here]) - making it:
{{{#!python
artist = models.ForeignKey(Musician, db_index=False)
}}}
4) Run `./manage.py makemigrations`
5) Run `./manage.py sqlmigrate <foo> 0002` to display the generated SQL
Expected:
Just the index is dropped.
{{{#!sql
BEGIN;
--
-- Alter field artist on album
--
DROP INDEX `foo_album_ca949605` ON `foo_album`;
COMMIT;
}}}
Actual:
The constraint is removed and re-added, which is time consuming since the
constraint is an index in itself.
{{{#!sql
BEGIN;
--
-- Alter field artist on album
--
ALTER TABLE `foo_album` DROP FOREIGN KEY
`foo_album_artist_id_66b4953c_fk_foo_musician_id`;
DROP INDEX `foo_album_ca949605` ON `foo_album`;
ALTER TABLE `foo_album` ADD CONSTRAINT
`foo_album_artist_id_66b4953c_fk_foo_musician_id` FOREIGN KEY
(`artist_id`) REFERENCES `foo_musician` (`id`);
COMMIT;
}}}
Additional context:
* The explicit `db_index=False` is needed to work around the duplicate
index issue discussed [https://groups.google.com/forum/#!topic/django-
developers/3ywugkcaxqs here].
* The duplicate index issue appears to be fixed on master (not sure by
what) causing this not to repro there.
* I'm presuming the duplicate index isn't severe enough to warrant
backporting, however dropping and re-adding the constraint seems pretty
bad, so perhaps more worthy of a backport to 1.10.x?
--
Ticket URL: <https://code.djangoproject.com/ticket/27558>
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