Unique fields that allow nulls in Django

Unique fields that allow nulls in Django

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:
    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?
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:


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

Answer 2:

** edit 11/30/2015: In python 3, the module-global __metaclass__ variable is no longer supported.
Additionaly, as of Django 1.10 the SubfieldBase class was deprecated:

from the docs:

django.db.models.fields.subclassing.SubfieldBase has been deprecated and will be removed in Django 1.10.
Historically, it was used to handle fields where type conversion was needed when loading from the database,
but it was not used in .values() calls or in aggregates. It has been replaced with from_db_value().
Note that the new approach does not call the to_python() method on assignment as was the case with SubfieldBase.

Therefore, as suggested by the from_db_value() documentation and this example, this solution must be changed to:

class CharNullField(models.CharField):

    Subclass of the CharField that allows empty strings to be stored as NULL.

    description = "CharField that stores NULL but returns ''."

    def from_db_value(self, value, expression, connection, contex):
        Gets value right out of the db and changes it if its ``None``.
        if value is None:
            return ''
            return value

    def to_python(self, value):
        Gets value right out of the db or an instance, and changes it if its ``None``.
        if isinstance(value, models.CharField):
            # If an instance, just return the instance.
            return value
        if value is None:
            # If db has NULL, convert it to ''.
            return ''

        # Otherwise, just return the value.
        return value

    def get_prep_value(self, value):
        Catches value right before sending to db.
        if value == '':
            # If Django tries to save an empty string, send the db None (NULL).
            return None
            # Otherwise, just pass the value.
            return value

I think a better way than overriding the cleaned_data in the admin would be to subclass the charfield – this way no matter what form accesses the field, it will “just work.” You can catch the '' just before it is sent to the database, and catch the NULL just after it comes out of the database, and the rest of Django won’t know/care. A quick and dirty example:

from django.db import models

class CharNullField(models.CharField):  # subclass the CharField
    description = "CharField that stores NULL but returns ''"
    __metaclass__ = models.SubfieldBase  # this ensures to_python will be called

    def to_python(self, value):
        # this is the value right out of the db, or an instance
        # if an instance, just return the instance
        if isinstance(value, models.CharField):
            return value 
        if value is None:  # if the db has a NULL (None in Python)
            return ''      # convert it into an empty string
            return value   # otherwise, just return the value

    def get_prep_value(self, value):  # catches value right before sending to db
        if value == '':   
            # if Django tries to save an empty string, send the db None (NULL)
            return None
            # otherwise, just pass the value
            return value  

For my project, I dumped this into an extras.py file that lives in the root of my site, then I can just from mysite.extras import CharNullField in my app’s models.py file. The field acts just like a CharField – just remember to set blank=True, null=True when declaring the field, or otherwise Django will throw a validation error (field required) or create a db column that doesn’t accept NULL.

Answer 3:

Because I am new to stackoverflow I am not yet allowed to reply to answers, but I would like to point out that from a philosophical point of view, I can’t agree with the most popular answer tot this question. (by Karen Tracey)

The OP requires his bar field to be unique if it has a value, and null otherwise. Then it must be that the model itself makes sure this is the case. It cannot be left to external code to check this, because that would mean it can be bypassed. (Or you can forget to check it if you write a new view in the future)

Therefore, to keep your code truly OOP, you must use an internal method of your Foo model. Modifying the save() method or the field are good options, but using a form to do this most certainly isn’t.

Personally I prefer using the CharNullField suggested, for portability to models I might define in the future.

Answer 4:

The quick fix is to do :

def save(self, *args, **kwargs):

    if not self.bar:
        self.bar = None

    super(Foo, self).save(*args, **kwargs)

Answer 5:

Another possible solution

class Foo(models.Model):
    value = models.CharField(max_length=255, unique=True)

class Bar(models.Model):
    foo = models.OneToOneField(Foo, null=True)

Answer 6:

I recently had the same requirement. Instead of subclassing different fields, I chose to override the save() metod on my model (named ‘MyModel’ below) as follows:

def save(self):
        """overriding save method so that we can save Null to database, instead of empty string (project requirement)"""
        # get a list of all model fields (i.e. self._meta.fields)...
        emptystringfields = [ field for field in self._meta.fields \
                # ...that are of type CharField or Textfield...
                if ((type(field) == django.db.models.fields.CharField) or (type(field) == django.db.models.fields.TextField)) \
                # ...and that contain the empty string
                and (getattr(self, field.name) == "") ]
        # set each of these fields to None (which tells Django to save Null)
        for field in emptystringfields:
            setattr(self, field.name, None)
        # call the super.save() method
        super(MyModel, self).save()    

Answer 7:

If you have a model MyModel and want my_field to be Null or unique, you can override model’s save method:

class MyModel(models.Model):
    my_field = models.TextField(unique=True, default=None, null=True, blank=True) 

    def save(self, **kwargs):
        self.my_field = self.my_field or None

This way, the field cannot be blank will only be non-blank or null. nulls do not contradict uniqueness

Answer 8:

For better or worse, Django considers NULL to be equivalent to NULL for purposes of uniqueness checks. There’s really no way around it short of writing your own implementation of the uniqueness check which considers NULL to be unique no matter how many times it occurs in a table.

(and keep in mind that some DB solutions take the same view of NULL, so code relying on one DB’s ideas about NULL may not be portable to others)