#24447: Migrations do not execute create_fk_sql() when adding a foreign key to a
field
----------------------------+--------------------
Reporter: ganwell | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 0 | UI/UX: 0
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Migrations failed when introducing foreign keys on existing fields.
Pull request: https://github.com/django/django/pull/4235
Branch with fix:
https://github.com/ganwell/django/tree/master_missing_create_fk_sql
Branch with unittest but no fix:
https://github.com/ganwell/django/tree/master_missing_create_fk_sql_fail
Consider following model:
{{{
class ForeignKeyTest(models.Model):
id = models.AutoField(primary_key=True)
customer = models.IntegerField()
class Customer(models.Model):
id = models.AutoField(primary_key=True)
}}}
Then it gets migrated to this:
{{{
class ForeignKeyTest(models.Model):
id = models.AutoField(primary_key=True)
customer = models.ForeignKey('Customer')
class Customer(models.Model):
id = models.AutoField(primary_key=True)
}}}
The second migration won't create the foreign key. This does not fail with
sqlite! Because these migrations are handled differently. It will fail
with MySQL and probably Postgres too. I wrote a unittest and tested it
with MySQL: master_missing_create_fk_sql_fail fails and
master_missing_create_fk_sql is ok.
The code didn't detect this case - if old_field.rel doesn't exist
alter_field() must always create the foreign key and ONLY if .rel exists
it must check .db_constraint, too. Since no .rel also means there was no
foreign key before.
I will of course redo the pull request to get clean history. I also fixed
this in 1.7.
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Ticket URL: <https://code.djangoproject.com/ticket/24447>
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