title: Django error on admin search: Cannot resolve keyword 'username' into field
body: <p>I am having a problem in my admin in Django. I have a list of users, I can add, edit and delete them without any problem. However, when I use the search bar to find specific users, i get the error:  </p>
  
  <pre><code>Cannot resolve keyword 'username' into field. Choices are: bookreader, date_joined, email, full_name, groups, id, is_active, is_staff, is_superuser, is_verified, last_login, logentry, password, profile, reading, user_permissions, usuário, virtualcurrency
  </code></pre>
  
  <p>I dont know why this is happening. Here is my User Model:</p>
  
  <pre><code>class User(AbstractBaseUser, PermissionsMixin):
      email = models.EmailField(_('email'), unique=True, blank=False)
      full_name = models.CharField(_('name'), max_length=30, blank=False)
  
      is_verified = models.BooleanField(_('verified'), default=False)
      is_staff = models.BooleanField(_('staff status'), default=False)
      is_active = models.BooleanField(_('active'), default=True)
      date_joined = models.DateTimeField(_('date joined'), default=now)
  
      objects = UserManager()
  
      USERNAME_FIELD = 'email'
  
      class Meta:
          verbose_name = _('user')
  
      def __str__(self):
          return self.email
  
      def get_short_name(self):
          return self.full_name.partition(' ')[0]
  
      def get_full_name(self):
          return self.full_name
  
      def send_verification_mail(self, request):
          if not self.pk:
              self.save()
  
          return send_template_mail(
              'Verificação de email',
              'users/user_verification_email.html',
              {
                  'user': self,
                  'verification_url': request.build_absolute_uri(
                      reverse('users:user_verify', kwargs={
                          'uidb64': urlsafe_base64_encode(force_bytes(self.pk)),
                          'token': default_token_generator.make_token(self)
                      })
                  )
              },
              [self.email]
          )
  </code></pre>
  
  <p>And the UserAdmin in admin.py:</p>
  
  <pre><code>class UserAdmin(UserAdmin_):
      list_display = ('email', 'full_name', 'is_staff')
      ordering = ('email',)
      fieldsets = (
          (None, {'fields': ('email', 'password')}),
          (_('Personal info'), {'fields': ('full_name',)}),
          (_('Permissions'), {'fields': (
              'is_active', 'is_verified', 'is_staff',
              'is_superuser', 'groups', 'user_permissions'
          )}),
          (_('Important dates'), {'fields': ('last_login', 'date_joined')}),
      )
      add_fieldsets = (
          (None, {
              'classes': ('wide',),
              'fields': ('email', 'password'),
          }),
      )
      form = UserChangeForm
      add_form = UserSignupForm
  
  
  admin.site.register(User, UserAdmin)
  </code></pre>
  
  <p>Why I am getting this error?</p>