content: from django.db import migrations, models
  
  
  def set_null_values(apps, schema_editor):
      """
      Replace empty strings with null values.
      """
      WirelessLAN = apps.get_model('wireless', 'WirelessLAN')
      WirelessLink = apps.get_model('wireless', 'WirelessLink')
      db_alias = schema_editor.connection.alias
  
      WirelessLAN.objects.using(db_alias).filter(auth_cipher='').update(auth_cipher=None)
      WirelessLAN.objects.using(db_alias).filter(auth_type='').update(auth_type=None)
      WirelessLink.objects.using(db_alias).filter(auth_cipher='').update(auth_cipher=None)
      WirelessLink.objects.using(db_alias).filter(auth_type='').update(auth_type=None)
      WirelessLink.objects.using(db_alias).filter(distance_unit='').update(distance_unit=None)
  
  
  class Migration(migrations.Migration):
      dependencies = [
          ('wireless', '0009_wirelesslink_distance'),
      ]
  
      operations = [
          migrations.AlterField(
              model_name='wirelesslan',
              name='auth_cipher',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.AlterField(
              model_name='wirelesslan',
              name='auth_type',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.AlterField(
              model_name='wirelesslink',
              name='auth_cipher',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.AlterField(
              model_name='wirelesslink',
              name='auth_type',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.AlterField(
              model_name='wirelesslink',
              name='distance_unit',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.RunPython(code=set_null_values, reverse_code=migrations.RunPython.noop),
      ]
  