title: Soft delete with unique constraint in Django
body: <p>I have models with this layout:</p>
  
  <pre><code>class SafeDeleteModel(models.Model):
      .....
      deleted = models.DateTimeField(editable=False, null=True)
      ......
  
  class MyModel(SafeDeleteModel):
      safedelete_policy = SOFT_DELETE
  
      field1 = models.CharField(max_length=200)
      field2 = models.CharField(max_length=200)
      field3 = models.ForeignKey(MyModel3)
      field4 = models.ForeignKey(MyModel4)
      field5 = models.ForeignKey(MyModel5)
  
      class Meta:
          unique_together = [['field2', 'field3', 'field4', 'deleted'],]
  </code></pre>
  
  <p>The scenario here is that I never want users to delete data. Instead a delete will just hide records. However, I still want all non-soft-deleted records to respect unique key constraints. Basically, I want to have as many duplicated deleted records, but only a single unique un-deleted record can exist. So I was thinking to include "deleted" field (provided by django-safedelete library), but the issue becomes that Django's unique checks fail with "psycopg2.IntegrityError: duplicate key value violates unique constraint" for ['field2', 'field3', 'field4', 'deleted'] because NULL is not "equal to" NULL and it yields false in PostgreSQL.</p>
  
  <p>Is there a way to enforce a unique_together constraint with the Django model layout as mine? Or is there a better idea to physically delete the record, then move it to an archive database, and if the user wants the record back, then software will look for the record in the archive and recreate it?</p>