[1.0.X] Fixed #9039 -- Don't perform unique checks on NULL values, since NULL != NULL in SQL.

    def validate_unique(self):

        from django.db.models.fields import FieldDoesNotExist

        # Gather a list of checks to perform. Since this is a ModelForm, some

        # fields may have been excluded; we can't perform a unique check on a

        # form that is missing fields involved in that check.

        # Gather a list of checks to perform. We only perform unique checks 

        # for fields present and not None in cleaned_data.  Since this is a 

        # ModelForm, some fields may have been excluded; we can't perform a unique 

        # check on a form that is missing fields involved in that check.  It also does

        # not make sense to check data that didn't validate, and since NULL does not 

        # equal NULL in SQL we should not do any unique checking for NULL values.

        unique_checks = []

        for check in self.instance._meta.unique_together[:]:

            fields_on_form = [field for field in check if field in self.fields]

            fields_on_form = [field for field in check if field in self.cleaned_data and not self.cleaned_data[field] is None]

            if len(fields_on_form) == len(check):

                unique_checks.append(check)

        form_errors = []

        # Gather a list of checks for fields declared as unique and add them to

        # the list of checks. Again, skip fields not on the form.

        # the list of checks. Again, skip empty fields and any that did not validate.

        for name, field in self.fields.items():

            try:

                f = self.instance._meta.get_field_by_name(name)[0]

            except FieldDoesNotExist:

                # This is an extra field that's not on the ModelForm, ignore it

                continue

            # MySQL can't handle ... WHERE pk IS NULL, so make sure we

            # don't generate queries of that form.

            is_null_pk = f.primary_key and self.cleaned_data[name] is None

            if name in self.cleaned_data and f.unique and not is_null_pk:

            if f.unique and name in self.cleaned_data and not self.cleaned_data[name] is None:

                unique_checks.append((name,))

        # Don't run unique checks on fields that already have an error.

        unique_checks = [check for check in unique_checks if not [x in self._errors for x in check if x in self._errors]]

        bad_fields = set()

        for unique_check in unique_checks:

            # Try to look up an existing object with the same values as this

   def __unicode__(self):

      return self.name

class Book(models.Model):

    title = models.CharField(max_length=40)

    author = models.ForeignKey(Writer, blank=True, null=True)

    special_id = models.IntegerField(blank=True, null=True, unique=True)

    class Meta:

        unique_together = ('title', 'author')

__test__ = {'API_TESTS': """

>>> from django import forms

>>> from django.forms.models import ModelForm, model_to_dict

>>> form.is_valid()

True

# Unique & unique together with null values

>>> class BookForm(ModelForm): 

...     class Meta: 

...        model = Book

>>> w = Writer.objects.get(name='Mike Royko')

>>> form = BookForm({'title': 'I May Be Wrong But I Doubt It', 'author' : w.pk})

>>> form.is_valid()

True

>>> form.save()

<Book: Book object>

>>> form = BookForm({'title': 'I May Be Wrong But I Doubt It', 'author' : w.pk})

>>> form.is_valid()

False

>>> form._errors

{'__all__': [u'Book with this Title and Author already exists.']}

>>> form = BookForm({'title': 'I May Be Wrong But I Doubt It'})

>>> form.is_valid()

True

>>> form.save()

<Book: Book object>

>>> form = BookForm({'title': 'I May Be Wrong But I Doubt It'})

>>> form.is_valid()

True

>>> form.save()

<Book: Book object>

# Choices on CharField and IntegerField

>>> class ArticleForm(ModelForm):

...     class Meta: