models.py

from django.db import models
from django.contrib.auth.models import User
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes.fields import GenericForeignKey


class Subscription(models.Model):
    user = models.ForeignKey(User,
                             related_name='subscription',
                             db_index=True,
                             on_delete=models.CASCADE)
    subscribe = models.BooleanField(default=True)
    target_ct = models.ForeignKey(ContentType,
                                  blank=True,
                                  null=True,
                                  related_name='target_obj',
                                  on_delete=models.CASCADE
                                  )
    target_id = models.PositiveIntegerField(null=True,
                                            blank=True,
                                            db_index=True)
    target = GenericForeignKey('target_ct', 'target_id')
    created = models.DateTimeField(auto_now_add=True, db_index=True)

    class Meta:
        ordering = ('-created', )