id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28193	Maximum length of database column in many-to-many through table is not updated if the foreign keyed model has a CharField primary key	James Hiew	nobody	"If you have a model `Foo`with a `CharField` primary key, and then another model `Bar` which has a `ManyToManyField` relationship with `Foo`, any changes to the `max_length` of `Foo`'s primary key is not reflected in the max string length of the corresponding Bar-Foo m2m table column. 

Starting with the below minimal example
{{{
from django.db import models


class Foo(models.Model):
    code = models.CharField(max_length=15, primary_key=True, unique=True, auto_created=False, editable=False)
    name = models.CharField(max_length=100)


class Bar(models.Model):
    foos = models.ManyToManyField(Foo)
}}}

Run `./manage.py makemigrations` then `./manage.py migrate`.

Change the `max_length` of `Foo.code` to a higher value e.g. 100
{{{
from django.db import models


class Foo(models.Model):
    code = models.CharField(max_length=100, primary_key=True, unique=True, auto_created=False, editable=False)
    name = models.CharField(max_length=100)


class Bar(models.Model):
    foos = models.ManyToManyField(Foo)
}}}
Run `./manage.py makemigrations` then `./manage.py migrate` once more.

The final `myapp_bar_foos` table DDL is:

{{{
CREATE TABLE ""myapp_bar_foos"" (""id"" integer NOT NULL PRIMARY KEY AUTOINCREMENT, ""bar_id"" integer NOT NULL REFERENCES ""myapp_bar"" (""id""), ""foo_id"" varchar(15) NOT NULL REFERENCES ""myapp_foo"" (""code""));
CREATE UNIQUE INDEX ""myapp_bar_foos_bar_id_foo_id_6037806e_uniq"" ON ""myapp_bar_foos"" (""bar_id"", ""foo_id"");
CREATE INDEX ""myapp_bar_foos_bar_id_f5103189"" ON ""myapp_bar_foos"" (""bar_id"");
CREATE INDEX ""myapp_bar_foos_foo_id_84900b21"" ON ""myapp_bar_foos"" (""foo_id"")
}}}

The `foo_id` column should have been updated to type `varchar(100)` but it remains as `varchar(15)`.

The problem is not noticeable when using SQLite (e.g. during local development), as SQLite doesn't seem to enforce the varchar length constraints, but Postgres does, so this issue is pernicious when moving to production."	Bug	closed	Migrations	1.11	Normal	duplicate	manytomanyfield, primary key, varchar, max length		Unreviewed	0	0	0	0	0	0