auto populating the fields in the models

Hello, please have a look at my models.py.

models.py:

class Thread(models.Model):
    pass

class ThreadParticipant(models.Model):
    thread = models.ForeignKey(Thread)
    user = models.ForeignKey(User)

class Message(models.Model):
    thread = models.ForeignKey(Thread)
    sent_date = models.DateTimeField(default=datetime.now)
    body = models.TextField()
    user = models.ForeignKey(User)

class MessageReadState(models.Model):
    message = models.ForeignKey(Message)
    user = models.ForeignKey(User)
    read_date = models.DateTimeField()

I am having two problems when I try to create a new message:

1. How do I auto populate the Thread with its primary key whenever I create a new message, without manually creating a new thread?
2. How to create a new user if the user is not in the the ThreadParticipant, or else don't create a new user?

I think I can solve this all in the views.py, but I think it will be better to solve this in the models.py. Please help me solve the problem. I would be very grateful. Thank you.

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