NOT NULL constraint failed: chat_message.chat_id?

Question

Django

I made a form for messages between users (similar to my own working form for comments). And for some reason this error comes out. There are enough links for such an error, but none of the suggestions helped me. I also tried to delete and reinstall the database with migrations - nothing. Prescribed in the chat field nall=True, blank = True, everything is also past.
My model (here, by the way, I will be grateful if someone throws at least something about how to correctly write logic for chats)

class Chat(models.Model):
    DIALOG = 'D'
    CHAT = 'C'
    CHAT_TYPE_CHOICES = (
        (DIALOG, 'Dialog'),
        (CHAT, 'Chat')
    )
    type = models.CharField( 'Тип',max_length=1,choices=CHAT_TYPE_CHOICES,default=DIALOG )
    slug = models.SlugField(unique=True)

    def get_absolute_url(self):
        return reverse('private_chat_url', kwargs={'slug': self.slug })


class Message(models.Model):
    chat = models.ForeignKey(Chat, verbose_name="Чат", on_delete=models.CASCADE)
    author = models.ForeignKey(User, verbose_name="Отправитель", on_delete=models.CASCADE)
    recipient = models.ForeignKey(User, related_name='received_messages', 
                           verbose_name="Получатель", on_delete=models.CASCADE, default=0)
    message = models.TextField("Сообщение")
    pub_date = models.DateTimeField('Дата сообщения', default=timezone.now)
    is_readed = models.BooleanField('Прочитано', default=False)

view

def private_chat(request, slug):
    pr_chat = get_list_or_404(Chat, slug__iexact=slug)
    body_chat = Message.objects.order_by('pk')

    if request.method == 'POST':
        message_form = MessageForm(request.POST)
        if message_form.is_valid():
            message_form.instance.author = request.user
            message_form.instance.post = pr_chat
            message_form.save()
            return redirect(pr_chat)
        else:
            HttpResponseNotFound("<h2>Введены неверные данные</h2>")
    else:
        message_form = MessageForm()
    return render(request, 'chat/private_chat.html', {'pr_chat': pr_chat,
                                                      'body_chat': body_chat,
                                                      'message_form': message_form,
                                                      })

the form

class MessageForm(forms.ModelForm):

    message = forms.CharField()

    class Meta:
        model = Message
        fields = ('message',)

Well, the form in the template

<form method="post">
    {% csrf_token %}
    {{ message_form }}
    <button type="submit" style="margin-top:1%; margin-bottom:1%; width:215px;" class="btn btn-outline-primary">Добавить</button>
</form>

Answer 1

message_form.instance.post = pr_chat
where is the post model field?

Answer 2

Did you write exactly nall=True in the model field or null =True? Only the last one is true. After changing the model, you need to generate a migration and roll it.

python manage.py makemigrations
python manage.py migrate