old_contents: from django.db import models
  
  # Create your models here.
  class VIPPackage(models.Model):
  	vpid = models.AutoField(primary_key = True)
  	nextPackage = models.ForeignKey("self", null=True, default = None)
  	start_date = models.DateTimeField()
  	days = models.IntegerField()
  	level = models.IntegerField()
  
  	class Meta:
  		db_table = "package_vip"
  
  class VIP(models.Model):
      """
          VIP System for user.
      """
  
      vid = models.AutoField(primary_key = True)
      headPackage = models.ForeignKey(VIPPackage, null=True, default = None)
  
      class Meta:
  
          db_table = "user_vip"
  
new_contents: from django.db import models
  
  # Create your models here.
  class VIPPackage(models.Model):
  	vpid = models.AutoField(primary_key = True)
  	nextPackage = models.ForeignKey("self", null=True, default = None)
  	start_date = models.DateTimeField(null=True, default = None)
  	days = models.IntegerField()
  	level = models.IntegerField()
  
  	class Meta:
  		db_table = "package_vip"
  
  class VIP(models.Model):
      """
          VIP System for user.
      """
  
      vid = models.AutoField(primary_key = True)
      headPackage = models.ForeignKey(VIPPackage, null=True, default = None)
  
      class Meta:
  
          db_table = "user_vip"
  
subject: allow null in vip models