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

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.

Simply use below code in models.py of any app

from django.contrib.auth.models import User
User._meta.get_field('email')._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

One possible way to do this is to have a pre-save hook on the User object and reject the save of the email already exists in the table.

Django does not allow direct editing User object but you can add pre_save signal and achieve unique email. for create signals u can follow https://docs.djangoproject.com/en/2.0/ref/signals/. then add the following to your signals.py

 @receiver(pre_save, sender=User)
def check_email(sender,instance,**kwargs):
    try:
        usr = User.objects.get(email=instance.email)
        if usr.username == instance.username:
            pass
        else:
            raise Exception('EmailExists')
    except User.DoesNotExist:
        pass

from an User inherited model, redefine the attribute correctly. It should work, as is it's not usefull to have that in django core because it's simple to do.