#29193: Altering a field with a unique constraint drops and rebuilds FKs to
other
fields in the table
---------------------------------+------------------------------------
Reporter: Jeremy Bowman | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.11
Severity: Release blocker | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
---------------------------------+------------------------------------
Comment (by Joshua Massover):
https://github.com/massover/bug_29193
The above is a small project to reproduce.
I could not reproduce it with the built in auth User model. I used a
custom User model. While using the built in auth User model, it looked
`_related_non_m2m_objects` did not return my related models because of
`related_objects`.
It looks like the code that changed the behavior is this
https://github.com/django/django/commit/c3e0adcad8d8ba94b33cabd137056166ed36dae0
#diff-b69190ab88f6c5737b2562d94d7bf36bR539
{{{
# Drop incoming FK constraints if the field is a primary key or
unique,
# which might be a to_field target, and things are going to
change.
drop_foreign_keys = (
(
(old_field.primary_key and new_field.primary_key)
(old_field.unique and new_field.unique)
) and old_type != new_type
)
}}}
Before it would only drop and recreate constraints if the field was a
foreign key. Now it drops and recreates all constraints if the field is
unique.
Naively removing the unique check that was added in
https://code.djangoproject.com/ticket/28305 will fix this problem. All the
tests pass. Looking back at the ticket I don't really understand the
intention, so this seems like an awful idea.
--
Ticket URL: <https://code.djangoproject.com/ticket/29193#comment:3>
Django <https://code.djangoproject.com/>
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