Allow case-insensitive usernames with Django & Postgres

admin.py

# Add the mixin to the default admin objects

from django.contrib.auth import get_user_model
from django.contrib.auth.admin import UserAdmin as ContribUserAdmin
from django.contrib.auth.forms import UserChangeForm as ContribUserChangeForm, \
    UserCreationForm as ContribUserCreationForm
from accounts.forms import CaseInsensitiveUsernameMixin  # adjust if forms.py is not in `accounts` app


class UserChangeForm(CaseInsensitiveUsernameMixin, ContribUserChangeForm):
    pass


class UserCreationForm(CaseInsensitiveUsernameMixin, ContribUserCreationForm):
    pass


class UserAdmin(ContribUserAdmin):
    add_form = UserCreationForm
    form = UserChangeForm
    save_on_top = True

admin.site.register(get_user_model(), UserAdmin)

forms.py

from django import forms
from django.contrib.auth import get_user_model
from django.utils.translation import ugettext_lazy as _


class CaseInsensitiveUsernameMixin():
    """
    Disallow a username with a case-insensitive match of existing usernames.
    Add this mixin to any forms that use the User object
    """
    def clean_username(self):
        username = self.cleaned_data.get('username')
        if get_user_model().objects.filter(username__iexact=username) \
                                   .exclude(pk=self.instance.pk).exists():
            raise forms.ValidationError(_(u'The username ‘{}’ is already in use.'.format(username)))
        return username

models.py

# For use with a custom user model, based on AbstractUser

from django.contrib.auth.models import AbstractUser, UserManager


class CaseInsensitiveUserManager(UserManager):
    def get_by_natural_key(self, username):
        """
        By default, Django does a case-sensitive check on usernames. This is Wrong™.
        Overriding this method fixes it.
        """
        return self.get(**{self.model.USERNAME_FIELD + '__iexact': username})


class User(AbstractUser):
    objects = CaseInsensitiveUserManager()
     # etc …