Unique fields that allow nulls in Django

Question

I have model Foo which has field bar. The bar field should be unique, but allow nulls in it, meaning I want to allow more than one record if bar field is null, but if it is not null the values must be unique.

Here is my model:

class Foo(models.Model):
    name = models.CharField(max_length=40)
    bar = models.CharField(max_length=40, unique=True, blank=True, null=True, default=None)

And here is the corresponding SQL for the table:

CREATE TABLE appl_foo
(
    id serial NOT NULL,
     "name" character varying(40) NOT NULL,
    bar character varying(40),
    CONSTRAINT appl_foo_pkey PRIMARY KEY (id),
    CONSTRAINT appl_foo_bar_key UNIQUE (bar)
)   

When using admin interface to create more than 1 foo objects where bar is null it gives me an error: "Foo with this Bar already exists."

However when I insert into database (PostgreSQL):

insert into appl_foo ("name", bar) values ('test1', null)
insert into appl_foo ("name", bar) values ('test2', null)

This works, just fine, it allows me to insert more than 1 record with bar being null, so the database allows me to do what I want, it's just something wrong with the Django model. Any ideas?

EDIT

The portability of the solution as far as DB is not an issue, we are happy with Postgres. I've tried setting unique to a callable, which was my function returning True/False for specific values of bar, it didn't give any errors, however seamed like it had no effect at all.

So far, I've removed the unique specifier from the bar property and handling the bar uniqueness in the application, however still looking for a more elegant solution. Any recommendations?

Answer 1

Django has not considered NULL to be equal to NULL for the purpose of uniqueness checks since ticket #9039 was fixed, see:

http://code.djangoproject.com/ticket/9039

The issue here is that the normalized "blank" value for a form CharField is an empty string, not None. So if you leave the field blank, you get an empty string, not NULL, stored in the DB. Empty strings are equal to empty strings for uniqueness checks, under both Django and database rules.

You can force the admin interface to store NULL for an empty string by providing your own customized model form for Foo with a clean_bar method that turns the empty string into None:

class FooForm(forms.ModelForm):
    class Meta:
        model = Foo
    def clean_bar(self):
        return self.cleaned_data['bar'] or None

class FooAdmin(admin.ModelAdmin):
    form = FooForm