Maximum length of Column in through table is not updated in migrations

[James Hiew]

If you have a model Foowith 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.