How to override username max_length in Django?

Question

I'm using Django 1.9, where usernames have a 30 character limit. In order to overcome this, I created a custom user model as such:

class User(AbstractUser):
    pass

# Override username max_length to allow long usernames.
User._meta.get_field('username').max_length = 255
User._meta.get_field('username').help_text = _(
    'Required. 255 characters or fewer. Letters, digits and @/./+/-/_ only.')

In the shell I can create users with names longer than 30 characters, but in the admin I cannot add users with long usernames or assign long usernames to existing users. I get:

Ensure this value has at most 30 characters (it has 43).

I noticed that Django Admin's UserCreateForm and UserChangeForm have Django's default User model explicitly set in their Meta options (which shouldn't be the case, there's a ticket regarding this), so I used custom forms like this:

from django.contrib.auth import get_user_model
from django.contrib.auth.admin import UserAdmin
from django.contrib.auth.forms import UserCreationForm, UserChangeForm

User = get_user_model()


class CustomUserCreationForm(UserCreationForm):
    class Meta(UserCreationForm.Meta):
        model = User


class CustomUserChangeForm(UserChangeForm):
    class Meta(UserChangeForm.Meta):
        model = User


class CustomUserAdmin(UserAdmin):
    form = CustomUserChangeForm
    add_form = CustomUserCreationForm

Still it didn't work. So I added a breakpoint to the CustomUserChangeForm's init, after calling super, and I got:

ipdb> self
<UserForm bound=True, valid=False, fields=(username;password;first_name;last_name;email;is_active;is_staff;is_superuser;groups;user_permissions)>
ipdb> self._meta.model
<class 'custom.models.User'>
ipdb> self.fields['username'].max_length
255
ipdb> self.fields['username'].validators[0].limit_value
255
ipdb> self.fields['username'].clean('a_username_with_more_than_thirty_characters')
u'a_username_with_more_than_thirty_characters'
ipdb> self.errors
{'username': [u'Ensure this value has at most 30 characters (it has 38).']}

I am mind blown. I don't know what I could possibly be missing. Any ideas?

Yes, and I checked in the debugger that the form was using the correct model. I edited the question and added this check. — Ariel, Sep 18 '18 at 06:48

score 2 · Accepted Answer · answered Sep 25 '18 at 14:08

Ok, I found out what the problem was. When Django's default username field is instantiated, it adds a MaxLengthValidator to its list of validators and uses the max_length argument to set its limit_value. When I monkey patched the User class, I overwrote the max_length and help_text of the username field, but I didn't take care of the validator. Adding this line under my custom User class fixed the issue:

User._meta.get_field('username').validators[1].limit_value = 255

The amount of time I spent debugging this is probably a good illustration of how monkey patching is usually a bad idea.

uedemir · Answer 2 · 2018-09-18T07:18:39.520

1

In other way, you can use your own custom model and fields with AbstractBaseUser.

from django.contrib.auth.models import AbstractBaseUser

class User(AbstractBaseUser):
    my_new_username = models.CharField(unique=True, max_length=50, verbose_name="Username")

    USERNAME_FIELD = 'my_new_username'

And the differences between AbstractBaseUser and AbstractUser explained here. Also there is other answer that might be help you.

edited Sep 18 '18 at 07:18

answered Sep 18 '18 at 07:04

uedemir

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I know, but it just seemed so tedious to go through all that configuration, explicitly configuring Django to use values that would have been the default anyway, just so I can change the max_length of the username… That's why I preferred to just inherit from AbstractUser directly. I don't understand why it shouldn't work the way I did it. – Ariel Sep 18 '18 at 08:13

How to override username max_length in Django?

2 Answers2