ForeignKey does not allow null values

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).

ForeignKey allows null values if this behavior was set. Your code will look like this:

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

You have to write null=True.

Note: after you change a model, you need to run python manage.py makemigrations yourappname and then python manage.py migrate

The solution with changing the model and allowing for null with:

blank=True,
null=True

wasn't enough for me.

I was able to solve the issue by setting required in the serializer to false, e.g.

field = MyModelSerializer(required=False)

as described here (Django ForeignKey field required despite blank=True and null=True).

I ended up doing both. Allowing for null with blank=True, null=True in the model and not requiring the field in the serializer by required=False

In my case, required=False wasn't enough. I needed allow_null in the serializer, per Django ForeignKey field required despite blank=True and null=True.