content: import django.db.models.deletion
  from django.db import migrations, models
  
  
  def migrate_contact_groups(apps, schema_editor):
      Contact = apps.get_model('tenancy', 'Contact')
      db_alias = schema_editor.connection.alias
  
      for contact in Contact.objects.using(db_alias).filter(group__isnull=False):
          contact.groups.add(contact.group)
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('tenancy', '0017_natural_ordering'),
      ]
  
      operations = [
          migrations.CreateModel(
              name='ContactGroupMembership',
              fields=[
                  ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False)),
              ],
              options={
                  'verbose_name': 'contact group membership',
                  'verbose_name_plural': 'contact group memberships',
              },
          ),
          migrations.RemoveConstraint(
              model_name='contact',
              name='tenancy_contact_unique_group_name',
          ),
          migrations.AddField(
              model_name='contactgroupmembership',
              name='contact',
              field=models.ForeignKey(
                  on_delete=django.db.models.deletion.CASCADE, related_name='+', to='tenancy.contact'
              ),
          ),
          migrations.AddField(
              model_name='contactgroupmembership',
              name='group',
              field=models.ForeignKey(
                  on_delete=django.db.models.deletion.CASCADE, related_name='+', to='tenancy.contactgroup'
              ),
          ),
          migrations.AddField(
              model_name='contact',
              name='groups',
              field=models.ManyToManyField(
                  blank=True,
                  related_name='contacts',
                  related_query_name='contact',
                  through='tenancy.ContactGroupMembership',
                  to='tenancy.contactgroup',
              ),
          ),
          migrations.AddConstraint(
              model_name='contactgroupmembership',
              constraint=models.UniqueConstraint(fields=('group', 'contact'), name='unique_group_name'),
          ),
          migrations.RunPython(code=migrate_contact_groups, reverse_code=migrations.RunPython.noop),
          migrations.RemoveField(
              model_name='contact',
              name='group',
          ),
      ]
  
  
  def oc_contact_groups(objectchange, reverting):
      for data in (objectchange.prechange_data, objectchange.postchange_data):
          if data is None:
              continue
          # Set the M2M field `groups` to a list containing the group ID
          data['groups'] = [data['group']] if data.get('group') else []
          data.pop('group', None)
  
  
  objectchange_migrators = {
      'tenancy.contact': oc_contact_groups,
  }
  