How to make email field unique in model User from contrib.auth in Django

Question

Caution: The code below was written for an older version of Django (before Custom User Models were introduced). It contains a race condition, and should only be used with a Transaction Isolation Level of SERIALIZABLE and request-scoped transactions.

Your code won't work, as the attributes of field instances are read-only. I fear it might be a wee bit more complicated than you're thinking.

If you'll only ever create User instances with a form, you can define a custom ModelForm that enforces this behavior:

from django import forms
from django.contrib.auth.models import User

class UserForm(forms.ModelForm):
    class Meta:
        model = User

    def clean_email(self):
        email = self.cleaned_data.get('email')
        username = self.cleaned_data.get('username')
        if email and User.objects.filter(email=email).exclude(username=username).exists():
            raise forms.ValidationError(u'Email addresses must be unique.')
        return email

Then just use this form wherever you need to create a new user.

BTW, you can use Model._meta.get_field('field_name') to get fields by name, rather than by position. So for example:

# The following lines are equivalent
User._meta.fields[4]
User._meta.get_field('email')

UPDATE

The Django documentation recommends you use the clean method for all validation that spans multiple form fields, because it's called after all the <FIELD>.clean and <FIELD>_clean methods. This means that you can (mostly) rely on the field's value being present in cleaned_data from within clean.

Since the form fields are validated in the order they're declared, I think it's okay to occasionally place multi-field validation in a <FIELD>_clean method, so long as the field in question appears after all other fields it depends on. I do this so any validation errors are associated with the field itself, rather than with the form.

What about using unique_together in a "different" way? So far it works for me.

class User(AbstractUser):
    ...
    class Meta(object):
        unique_together = ('email',)

Simply use below code in models.py of any app

from django.contrib.auth.models import User
User._meta.get_field('email')._unique = True

In settings module:

# Fix: username length is too small,email must be unique
from django.contrib.auth.models import User, models
User._meta.local_fields[1].__dict__['max_length'] = 75
User._meta.local_fields[4].__dict__['_unique'] = True

It's amazing, but I found a best solution for me!

django-registration have form with checking uniqueness of email field: RegistrationFormUniqueEmail

example of usage here