#26384: Django 1.9 regression in migrations due to lazy model operations
refactor
---------------------------------+------------------------------------
Reporter: brian | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.9
Severity: Release blocker | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
---------------------------------+------------------------------------
Changes (by AlexHill):
* has_patch: 0 => 1
Comment:
The model in Tim's test case looks like this:
{{{
#!python
class Foo(models.Model):
# album_id = models.AutoField(primary_key=True) (renamed to `id` in
migration 2)
parent = models.ForeignKey('self', null=True, blank=True)
}}}
The problematic migration wants to rename `Foo.album_id` to `Foo.id`. To
perform this kind of schema alteration under SQLite, we have to create a
new model with updated fields, and from that model make a new table, copy
the data, then remove the old table. The action happens in
[https://github.com/django/django/blob/67cf5efa31acb2916034afb15610b700695dfcb0/django/db/backends/sqlite3/schema.py#L68
db.backends.sqlite3.schema.DatabaseSchemaEditor._remake_table()].
The bug occurs because only the AutoField is changed, while the ForeignKey
is considered not to have changed and is copied directly from the pre-
migration model. In fact the ForeignKey, in its `remote_field.field_name`,
holds a copy of the AutoField's field name, so it needs to be updated too
to ensure the temporary model is internally consistent.
To fix the bug we can re-create a model's self-referential related fields
using Field.clone() rather than just using the old model's field
instances.
I've opened a PR at https://github.com/django/django/pull/6355.
--
Ticket URL: <https://code.djangoproject.com/ticket/26384#comment:9>
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