Django: import auth user to the model

from django.contrib.auth.models import User

You missed the models - and user is capitalized.

If you use a custom user model you should use:

from django.contrib.auth import get_user_model
User = get_user_model()

More details can be found in the docs.

Changed in Django 1.11:

The ability to call get_user_model() at import time was added.

If you're using a custom User model, do the following to reference it:

from django.contrib.auth import get_user_model
User = get_user_model()

Or if using it in foreign key or many-to-many relations:

from django.conf import settings
....
user = models.ForeignKey(settings.AUTH_USER_MODEL)

docs

AUTH_USER_MODEL is a good solution. here is the complete solution as per the question.

from django.db import models
from django.conf import settings

class Workers(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL)
    work_group = models.CharField(max_length=20)
    card_num = models.IntegerField()

    def __unicode__(self):
        return self.user.id