content: from django.db import migrations, models
  
  
  def set_null_values(apps, schema_editor):
      """
      Replace empty strings with null values.
      """
      CustomFieldChoiceSet = apps.get_model('extras', 'CustomFieldChoiceSet')
      db_alias = schema_editor.connection.alias
  
      CustomFieldChoiceSet.objects.using(db_alias).filter(base_choices='').update(base_choices=None)
  
  
  class Migration(migrations.Migration):
      dependencies = [
          ('extras', '0121_customfield_related_object_filter'),
      ]
  
      operations = [
          migrations.AlterField(
              model_name='customfieldchoiceset',
              name='base_choices',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.RunPython(code=set_null_values, reverse_code=migrations.RunPython.noop),
      ]
  