Django 1.11 many to many does not appear in django admin - django

Question

Hi i have a django model for notification which have a many-to-many relation but nothing appears in django admin ( all fields do not appear)
class Notification(models.Model):
"""send notification model"""
title = models.CharField(max_length=200)
text = models.TextField(null=True, blank=True)
device = models.ManyToManyField(Device, null=True, blank=True)
country = models.ManyToManyField(Country, null=True, blank=True)
sent = models.BooleanField(default=False)
when i open django admin for this model and press add notification this is what happens (nothing appears)
Country and Device Code
class Device(models.Model):
"""Store device related to :model:`accounts.User`."""
user = models.OneToOneField(User, related_name='device', on_delete=models.CASCADE)
model = models.CharField(max_length=200, blank=True, null=True)
player_id = models.CharField(max_length=200, blank=True, null=True)
class Meta:
verbose_name = 'Device'
verbose_name_plural = 'Devices'
def __str__(self):
return self.model
class Country(models.Model):
name = models.CharField(max_length=255)
def __str__(self):
return self.name
Admin.py
from django.contrib import admin
from .models import Notification
admin.site.register(Notification)
Edit:
Thank you all the problem is solved
The problem was caused by some entries in device model that did have None in the model field so there was a problem displaying it correctly.

Answer 1

According to https://code.djangoproject.com/ticket/2169 :
When a class has a field that isn't shown in the admin interface but
must not be blank, it's impossible to add a new one. You get a cryptic
"Please correct the error below." message with no shown errors. The
error message should probably say something about a hidden field.
Now ManyToManyFields don't need null=True, try removing those statements and see if you get an improvement.
Also, try adding the Country and Device models in admin.py so admin can see them and display them.

Answer 2

https://docs.djangoproject.com/en/1.11/ref/contrib/admin/#working-with-many-to-many-models
Define an inline for the many-to-manys in admin.py:
from django.contrib import admin
class DeviceInline(admin.TabularInline):
model = Notification.device.through
class CountryInline(admin.TabularInline):
model = Notification.country.through
class NotificationAdmin(admin.ModelAdmin):
inlines = [
DeviceInline, CountryInline
]
exclude = ("device", "country")