save() prohibited to prevent data loss due to unsaved related object

Answered - The problem arose from django not saving empty or unchanged forms. This led to null fields on those unsaved forms. Problem was fixed by allowing null fields on foreign keys, as a matter of fact -- all fields. That way, empty or unchanged forms did not return any errors on save.

FYI: Refer to @wolendranh answer.

This was introduced in Django 1.8. Previously you could assign not saved instance to One-To-One relation and in case of fail it was silently skipped. Starting from Django 1.8 you will get error message in this case. Check a documentation of Django 1.7 -> 1.8 upgrade.

It says:

Assigning unsaved objects to a ForeignKey, GenericForeignKey, and OneToOneField now raises a ValueError.

If you are interested in more details, you can check save method in django.db.models.base: Some part of it:

for field in self._meta.concrete_fields:
    if field.is_relation:
        # If the related field isn't cached, then an instance hasn't
        # been assigned and there's no need to worry about this check.
        try:
            getattr(self, field.get_cache_name())
        except AttributeError:
            continue
        obj = getattr(self, field.name, None)
        # A pk may have been assigned manually to a model instance not
        # saved to the database (or auto-generated in a case like
        # UUIDField), but we allow the save to proceed and rely on the
        # database to raise an IntegrityError if applicable. If
        # constraints aren't supported by the database, there's the
        # unavoidable risk of data corruption.
        if obj and obj.pk is None:
            raise ValueError(
                "save() prohibited to prevent data loss due to "
                "unsaved related object '%s'." % field.name
            )

Last 5 rows are where this error is raised. basically your related obj which is not saved will have obj.pk == None and ValueError will be raised.

it is simple:

p3 = Place(name='Demon Dogs', address='944 W. Fullerton')   
p3.save() # <--- you need to save the instance first, and then assign
Restaurant.objects.create(
    place=p3, serves_hot_dogs=True, serves_pizza=False
)