Can django's auth_user.username be varchar(75)? How could that be done?

There's a way to achieve that without touching the core model, and without inheritance, but it's definitely hackish and I would use it with extra care.

If you look at Django's doc on signals, you'll see there's one called class_prepared, which is basically sent once any actual model class has been created by the metaclass. That moment is your last chance of modifying any model before any magic takes place (ie: ModelForm, ModelAdmin, syncdb, etc...).

So the plan is simple, you just register that signal with a handler that will detect when it is called for the User model, and then change the max_length property of the username field.

Now the question is, where should this code lives? It has to be executed before the User model is loaded, so that often means very early. Unfortunately, you can't (django 1.1.1, haven't check with another version) put that in settings because importing signals there will break things.

A better choice would be to put it in a dummy app's models module, and to put that app on top of the INSTALLED_APPS list/tuple (so it gets imported before anything else). Here is an example of what you can have in myhackishfix_app/models.py :

from django.db.models.signals import class_prepareddef longer_username(sender, *args, **kwargs):    # You can't just do `if sender == django.contrib.auth.models.User`    # because you would have to import the model    # You have to test using __name__ and __module__    if sender.__name__ == "User" and sender.__module__ == "django.contrib.auth.models":        sender._meta.get_field("username").max_length = 75class_prepared.connect(longer_username)

That will do the trick.

A few notes though:

• You might want to change also the help_text of the field, to reflect the new maximum length
• If you want to use the automatic admin, you will have to subclass UserChangeForm, UserCreationForm and AuthenticationForm as the maximum length is not deduced from the model field, but directly in the form field declaration.

If you're using South, you can create the following migration to change the column in the underlying database:

import datetimefrom south.db import dbfrom south.v2 import SchemaMigrationfrom django.db import modelsclass Migration(SchemaMigration):    def forwards(self, orm):        # Changing field 'User.username'        db.alter_column('auth_user', 'username', models.CharField(max_length=75))    def backwards(self, orm):        # Changing field 'User.username'        db.alter_column('auth_user', 'username', models.CharField(max_length=35))    models = { # ... Copy the remainder of the file from the previous migration, being sure # to change the value for auth.user / usename / maxlength

Based on Clément and Matt Miller's great combined answer above, I've pulled together a quick app that implements it. Pip install, migrate, and go. Would put this as a comment, but don't have the cred yet!

https://github.com/GoodCloud/django-longer-username

EDIT 2014-12-08

The above module is now deprecated in favor of https://github.com/madssj/django-longer-username-and-email

Updated solution for the Django 1.3 version (without modifying manage.py):

Create new django-app:

monkey_patch/    __init__.py    models.py

Install it as first: (settings.py)

INSTALLED_APPS = (    'monkey_patch',     #...)

Here is models.py:

from django.contrib.auth.models import Userfrom django.core.validators import MaxLengthValidatorNEW_USERNAME_LENGTH = 300def monkey_patch_username():    username = User._meta.get_field("username")    username.max_length = NEW_USERNAME_LENGTH    for v in username.validators:        if isinstance(v, MaxLengthValidator):            v.limit_value = NEW_USERNAME_LENGTHmonkey_patch_username()