ForeignKey does not allow null values

Question

I am using the Django REST Framework 2.0.

Here is my model class:

class Mission(models.Model):
  assigned_to = models.ForeignKey('auth.User',
                                   related_name='missions_assigned',
                                   blank = True)

Here is my view class:

class MissionList(generics.ListCreateAPIView):
    model = Mission
    serialize_class = MissionSerializer

1. The multipart form is rendered in the browser with empty choice for assigned_to field.

2. When posting raw JSON, I get the following error message:

Cannot assign None: "Mission.assigned_to" does not allow null values.

Answer 1

The blank option is used in the form validation, and the null is used when writing to database.

So you might add null=True to that field.

EDIT: continue the comment

Considering the two steps when saving object:

1. Validator(controlled by blank)
2. Database limitation(controlled by null)

For default option, take IntegerField for example,
default=5, blank=True, null=False, pass (1) even if you didn't assign a value(having blank=True), pass (2) because it has a default value(5) and writes 5 instead of None to DB.
blank=True, null=False, which pass (1) but not (2), because it attempts to write None to DB.

Thus, if you want to make a field optional, use either default=SOMETHING, blank=True, null=False or blank=True, null=True.

Another exception is the string-like field, such as CharField.
It's suggested that use the blank=True alone, leaving null=False behind.
This makes a field either a string(>=1 char(s)) or a empty string('', with len()==0), and never None.

The reason is that when null=True is set, there will be two possible value for the state "unset": empty string and None, which is confusing(and might causing bugs).