#31064: Migration doesn't detect precision changes in fields that ManyToMany
points
to.
-----------------------------+------------------------------------
Reporter: zapililirad | Owner: Dart
Type: Bug | Status: assigned
Component: Migrations | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
Sanskar it's possible the issue is only present on SQLite because it deals
with column alterations in a different way since it doesn't support `ALTER
TABLE`.
The idea is that is you have the follow models
{{{#!python
class Author(models.Model):
pass
class Book(models.Model):
isbn = models.CharField(max_length=10, primary_key=True)
authors = models.ManyToManyField(Author)
}}}
Django will automatically created a `trough` model
{{{#!python
# Automatically created by Django
class Book_Authors(models.Model):
book= models.ForeignKey(Book) # This is a
models.CharField(max_length=10) referring to Book.isbn
author = models.ForeignKey(Author)
}}}
Now the reported issue is that if you If you change `Book.isbn.max_length`
to 13 the `Book_Authors.book` field in the intermediary table that backs
the `Book.authors` many-to-many relationship currently doesn't change to
`max_length=13`.
[https://github.com/django/django/blob/a4881f5e5d7ee38b7e83301331a0b4962845ef8a/django/db/backends/base/schema.py#L757
The re-pointing logic currently lives] in
`BaseDatabaseSchemaEditor._alter_field` but it's possible that it's only
[https://github.com/django/django/blob/a4881f5e5d7ee38b7e83301331a0b4962845ef8a/django/db/backends/sqlite3/schema.py#L348-L365
an issue with SQLite schema editor].
By a quick glance at the code it looks like it could be a SQLite only
issue due to
[https://github.com/django/django/blob/a4881f5e5d7ee38b7e83301331a0b4962845ef8a/django/db/backends/sqlite3/schema.py#L364
this particular line].
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Ticket URL: <https://code.djangoproject.com/ticket/31064#comment:4>
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