Auto-migrations fail on postgres when resizing Charfield and setting default value longer than old constraint

[shadytradesman]

Migration to create the field

# Generated by Django 2.2.12 on 2020-05-29 19:06

import cells.models
from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('cells', '0003_auto_20200529_1832'),
    ]

    operations = [
        migrations.AddField(
            model_name='cell',
            name='invite_link_secret_key',
            field=models.CharField(default=cells.models.random_string, max_length=7),
        ),
    ]

Migration to resize the field and add a default:

# Generated by Django 2.2.12 on 2020-05-29 23:56

import cells.models
from django.db import migrations, models


class Migration(migrations.Migration):

    dependencies = [
        ('cells', '0004_auto_20200529_2006'),
    ]

    operations = [
        migrations.AlterField(
            model_name='cell',
            name='invite_link_secret_key',
            field=models.CharField(default=cells.models.random_string, max_length=64),
        ),
    ]

Here is the random string method:

def random_string():
    return hashlib.sha224(bytes(random.randint(1, 99999999))).hexdigest()

This migration appeared to work on mysql when I was developing locally, but it may be because I updated the random_string() function to provide a string that was longer after I ran the migration. It failed when I ran it against my prod Postgres 9.5.15 database.