class AbstractUser(AbstractBaseUser, PermissionsMixin):
    """
    An abstract base class implementing a fully featured User model with
    admin-compliant permissions.

    Username and password are required. Other fields are optional.
    """
    username_validator = UnicodeUsernameValidator()

    username = models.CharField(
        _('username'),
        max_length=150,
        unique=True,
        help_text=_('Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.'),
        validators=[username_validator],
        error_messages={
            'unique': _("A user with that username already exists."),
        },
    )
    first_name = models.CharField(_('first name'), max_length=30, blank=True)
    last_name = models.CharField(_('last name'), max_length=150, blank=True)
    email = models.EmailField(_('email address'), blank=True)
    is_staff = models.BooleanField(
        _('staff status'),
        default=False,
        help_text=_('Designates whether the user can log into this admin site.'),
    )
    is_active = models.BooleanField(
        _('active'),
        default=True,
        help_text=_(
            'Designates whether this user should be treated as active. '
            'Unselect this instead of deleting accounts.'
        ),
    )
    date_joined = models.DateTimeField(_('date joined'), default=timezone.now)

    objects = UserManager()

    EMAIL_FIELD = 'email'
    USERNAME_FIELD = 'username'
    REQUIRED_FIELDS = ['email']

    class Meta:
        verbose_name = _('user')
        verbose_name_plural = _('users')
        abstract = True # abstract 옵션이 True로 설정된 클래스는 makemigrations 커맨드 실행 시에 무시한다.

    def clean(self):
        super().clean()
        self.email = self.__class__.objects.normalize_email(self.email)

    def get_full_name(self):
        """
        Return the first_name plus the last_name, with a space in between.
        """
        full_name = '%s %s' % (self.first_name, self.last_name)
        return full_name.strip()

    def get_short_name(self):
        """Return the short name for the user."""
        return self.first_name

    def email_user(self, subject, message, from_email=None, **kwargs):
        """Send an email to this user."""
        send_mail(subject, message, from_email, [self.email], **kwargs)


class User(AbstractUser):
    """
    Users within the Django authentication system are represented by this
    model.

    Username and password are required. Other fields are optional.
    """
    class Meta(AbstractUser.Meta):
        swappable = 'AUTH_USER_MODEL'

# user/models.py
from django.db import models
from django.contrib.auth.models import AbstractBaseUser, PermissionsMixin, UserManager

class User(AbstractBaseUser, PermissionsMixin):
    username = models.CharField('아이디', max_length=10, unique=True)
    password = models.CharField('비밀번호', max_length=10)
    email = models.EmailField('이메일', unique=True)
    is_staff = models.BooleanField('스태프 권한', default=False)
    is_active = models.BooleanField('사용여부', default=True)
    created_at = models.DateTimeField('가입일', auto_now_add=True)

    objects = UserManager()

    USERNAME_FIELD = 'username' # 아이디를 사용자 식별자로 설정
    REQUIRED_FIELDS = ['email'] # 필수입력값

# conf/settings.py
INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'polls',
    'board',
    'user',
]

AUTH_USER_MODEL = 'user.User'