class User(models.Model):
   first_name = models.CharField(max_length=255)
   last_name = models.CharField(max_length=255)

class Blog(models.Model):
   title = models.CharField(max_length=255)
   author = models.ForeignKey(User,on_delete=models.CASCADE)
   created = models.DateTimeField(default=timezone.now())

   subscribers = models.ManyToManyField(User, related_name='subscriptions')

class Topic(models.Model):
   title = models.CharField(max_length=255)
   blog = models.ForeignKey(Blog,on_delete=models.CASCADE)
   author = models.ForeignKey(User,on_delete=models.CASCADE)
   created = models.DateTimeField(default=timezone.now())
   likes = models.ManyToManyField(User, related_name='likes')