models.py

from django.contrib.auth.models import User
from django.db import models
from django.utils.html import mark_safe
from django.utils.text import Truncator

from markdown import markdown


class Board(models.Model):
    name = models.CharField(max_length=30, unique=True)
    description = models.CharField(max_length=100)

    def __str__(self):
        return self.name

    def get_posts_count(self):
        return Post.objects.filter(topic__board=self).count()

    def get_last_post(self):
        return Post.objects.filter(topic__board=self).order_by('-created_at').first()


class Topic(models.Model):
    subject = models.CharField(max_length=255)
    last_updated = models.DateTimeField(auto_now_add=True)
    board = models.ForeignKey(Board, related_name='topics')
    starter = models.ForeignKey(User, related_name='topics')
    views = models.PositiveIntegerField(default=0)

    def __str__(self):
        return self.subject


class Post(models.Model):
    message = models.TextField(max_length=4000)
    topic = models.ForeignKey(Topic, related_name='posts')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(null=True)
    created_by = models.ForeignKey(User, related_name='posts')
    updated_by = models.ForeignKey(User, null=True, related_name='+')

    def __str__(self):
        truncated_message = Truncator(self.message)
        return truncated_message.chars(30)

    def get_message_as_markdown(self):
        return mark_safe(markdown(self.message, safe_mode='escape'))

In case anyone is using python 3.6.5 and you get the following error after making changes:

TypeError: init() missing 1 required positional argument: 'on_delete'

Just make sure you add **on_delete=models.DO_NOTHING ** or on_delete=models.CASCADE
to the lines that are being flagged in your terminal, it should be the ones with the models.foreignKeys .

for example:

class Topic(models.Model):
subject = models.CharField(max_length=255)
last_updated = models.DateTimeField(auto_now_add=True)
board = models.ForeignKey(Board, related_name='topics', on_delete=models.DO_NOTHING)
starter = models.ForeignKey(User, related_name='topics', on_delete=models.DO_NOTHING)
views = models.PositiveIntegerField(default=0)

def __str__(self):
    return self.subject

class Post(models.Model):
message = models.TextField(max_length=4000)
topic = models.ForeignKey(Topic, related_name='posts', on_delete=models.DO_NOTHING)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(null=True)
created_by = models.ForeignKey(User, related_name='posts', on_delete=models.DO_NOTHING)
updated_by = models.ForeignKey(User, null=True, related_name='+',on_delete=models.CASCADE)