content: from django.db import migrations, models
  
  
  def set_null_values(apps, schema_editor):
      """
      Replace empty strings with null values.
      """
      Circuit = apps.get_model('circuits', 'Circuit')
      CircuitGroupAssignment = apps.get_model('circuits', 'CircuitGroupAssignment')
      CircuitTermination = apps.get_model('circuits', 'CircuitTermination')
      db_alias = schema_editor.connection.alias
  
      Circuit.objects.using(db_alias).filter(distance_unit='').update(distance_unit=None)
      CircuitGroupAssignment.objects.using(db_alias).filter(priority='').update(priority=None)
      CircuitTermination.objects.using(db_alias).filter(cable_end='').update(cable_end=None)
  
  
  class Migration(migrations.Migration):
      dependencies = [
          ('circuits', '0045_circuit_distance'),
      ]
  
      operations = [
          migrations.AlterField(
              model_name='circuit',
              name='distance_unit',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.AlterField(
              model_name='circuitgroupassignment',
              name='priority',
              field=models.CharField(blank=True, max_length=50, null=True),
          ),
          migrations.AlterField(
              model_name='circuittermination',
              name='cable_end',
              field=models.CharField(blank=True, max_length=1, null=True),
          ),
          migrations.RunPython(code=set_null_values, reverse_code=migrations.RunPython.noop),
      ]
  