Django conditional unique together

Overriding validate_unique to check the uniqueness if is_deleted is False is more appropriate:

...

def validate_unique(self, exclude=None):
    if not self.is_deleted and \
       LibraryEntry.objects.exclude(pk=self.pk).filter(host_lib_song_id=self.host_lib_song_id, owning_user=self.owning_user).exists():
        raise ValidationError('Some error message about uniqueness required')
    super(LibraryEntry, self).validate_unique(exclude=exclude)

You cannot express this through the Meta.unique_together constraint, but through django's model validation:

class LibraryEntry(models.Model):
    def clean(self):
        from django.core.exceptions import ValidationError
        try:
            # try to find a duplicate entry and exclude 'self'
            duplicate = LibraryEntry.objects.exclude(pk=self.pk)\
                .get(owning_user=self.owning_user, 
                     host_lib_song_id=self.host_lib_song_id,
                     is_deleted=False)
            raise ValidationError('Library Entry already exists!')
        except: LibraryEntry.DoesNotExist:
            # no duplicate found
            pass