尝试将新的 model 和字段添加到 Django 中的现有数据中

Question

I'm learning Django and making a kind of social network and am trying to add a model and field to my models. The following post perfectly explains what I'm trying to do...

Django - Private messaging conversation view

I'm trying to add the model and field after I already have some messages saved. I'm making this for learning purposes for now, so deleting the existing messages and starting over would probably solve the problem, but I'm trying to learn how to navigate this because next time I could have actual users and deleting the messages might not be possible.

Here are my models:

from django.db import models
from django.contrib.auth.models import User

# Create your models here.

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    city = models.CharField(max_length=60)
    country = models.CharField(max_length=60)
    skillstolearn = models.CharField(max_length=200)
    skillstoteach = models.CharField(max_length=200)
    description = models.TextField()

    def __str__(self):
        return self.user.username

class Conversation(models.Model):
    participants = models.ManyToManyField(User)
    
    def __str__(self):
        return self.participants

class Message(models.Model):
    sender = models.ForeignKey(User, on_delete=models.CASCADE, related_name='sender')
    receiver = models.ForeignKey(User, on_delete=models.CASCADE, related_name='receiver')
    content = models.TextField()
    conversation = models.ForeignKey(Conversation, default=None, on_delete=models.CASCADE)
    timestamp = models.DateTimeField()

    def __str__(self):
        return self.sender.username + ' to ' + self.receiver.username

Before trying to make the current migrations, the file did not have the Conversation model nor the conversation field in the Message model.

When I try to migrate, I get:

django.db.utils.IntegrityError: NOT NULL constraint failed: new__thecode_message.conversation_id

How do I get this working and how do I deal with this if I need to change a model, a field, et cetera after already having users?

Answer 1

If you add new field to model, you cannot ask to fill existing objects with None value, when you do not accept None values. Add null=True to conversation field:

class Message(models.Model):
    ...
    conversation = models.ForeignKey(Conversation, default=None, on_delete=models.CASCADE, null=True)
    ...

then makemigrations and migrate . It should work. If not, just delete probably last migration file (with that field included) in folder migrations .