django-allauth set username the same as email

profiles.models.py (custom user model)

from allauth.socialaccount.adapter import DefaultSocialAccountAdapter

class CustomSocialAccountAdapter(DefaultSocialAccountAdapter):

    def populate_user(self, request, sociallogin, data):
        user = super().populate_user(request, sociallogin, data)
        user.username = user.email
        return user

settings.py

SOCIALACCOUNT_ADAPTER = "profiles.models.CustomSocialAccountAdapter"

I do exactly what you want to do with a signal on User pre_save.

Your settings look ok, so if you add the following code in somewhere like for example core.models.py it will work as you need:

@receiver(pre_save, sender=User)
def update_username_from_email(sender, instance, **kwargs):
    user_email = instance.email
    username = user_email[:30]
    n = 1
    while User.objects.exclude(pk=instance.pk).filter(username=username).exists():
        n += 1
        username = user_email[:(29 - len(str(n)))] + '-' + str(n)
    instance.username = username

The reason I do it with a signal is that I want that every time User is saved, the username is updated. You could check if the e-mail has changed update the username only in that case.

Then I limit the username to the first 30 characters of email (default max lenght of username is 30 chars):

username = user_email[:30]

You could also change the max lenght of username, but in my case I prefered to use the default lenght.

Since I make this, it could happen that there are repeated usernames. To avoid repeated usernames, in case that the resulting username after limiting it to 30 characters already exists, I put -2, -3... at the end to make the username unique:

n = 1
while User.objects.exclude(pk=instance.pk).filter(username=username).exists():
    n += 1
    username = user_email[:(29 - len(str(n)))] + '-' + str(n)
instance.username = username

I hope this solution helps you!