Brittle unique_for best practice

Brittle unique_for

unique_for_date/month/year is very brittle and over time will likely be a source for unexpected behaviour.

unique_for_date/month/year prevents entry of two records with the same value on the same date or month or year.

However, this feature is unfortunately brittle:

This constraint is not enforced by the database.
It is checked during Model.validate_unique() and so will not occur if Model.save() is called without first calling Model.validate_unique().
The constraint will not be checked even if Model.validate_unique() is called when using a ModelForm that does not include a field involved in the check.
Only the date portion of the field will be considered, even when validating a DateTimeField.

Most of these problems can be mitigated by moving the validation to Model.save().

If our GitHub code review bot spots this issue in your pull request it gives this advice:

django-doctorbotsuggested changes just now

models.py

1	+	class ExampleModel(models.Model):
2	+	date = models.DateField()
3	+	foo = models.CharField(unique_for_date='date')

unique_for_date/month/year is very brittle and over time will likely be a source for unexpected behaviour.

Suggested changes

--
+    foo = models.CharField(unique_for_date='date')
-+
+    foo = models.CharField()
-+
-+
+    def save(self, *args, **kwargs):
-+
+        # more verbose, but less brittle than unique_for_date
-+
+        if self.objects.filter(date=self.date, foo=self.foo).exists():
-+
+            raise ValidationError({'foo': 'Must be unique for date.'})
-+
+        return super().save(*args, **kwargs)

Update models.py