foreign key in testing django with factory boy doesn't work

I have a problem with foreign key in library factory boy. My test doesn't execute I thinking that the problem in foreign key.

I try to test user model which is in user.models that is how my code look like

class Task(models.Model):
    title = models.CharField(max_length=255, verbose_name='Заголовок')
    description = models.CharField(max_length=255, verbose_name='Описание')
    cost = models.DecimalField(max_digits=7, decimal_places=2, default=0, verbose_name='Цена')
    assignee = models.ForeignKey('users.User', related_name='assignee', null=True, verbose_name='Исполнитель')
    created_by = models.ForeignKey('users.User', related_name='created_by', verbose_name='Кем был создан')

    def __str__(self):
        return self.title

I test it with factory boy that is how my factory boy class looks like

class UserFactoryCustomer(factory.Factory):

    class Meta:
        model = User

    first_name = 'Ahmed'
    last_name = 'Asadov'
    username = factory.LazyAttribute(lambda o: slugify(o.first_name + '.' + o.last_name))
    email = factory.LazyAttribute(lambda a: '{0}.{1}@example.com'.format(a.first_name, a.last_name).lower())
    user_type = 1
    balance = 10000.00

class UserFactoryExecutor(factory.Factory):

    class Meta:
        model = User

    first_name = 'Uluk'
    last_name = 'Djunusov'
    username = factory.LazyAttribute(lambda o: slugify(o.first_name + '.' + o.last_name))
    email = factory.LazyAttribute(lambda a: '{0}.{1}@example.com'.format(a.first_name, a.last_name).lower())
    user_type = 2
    balance = 5000.00


class TaskFactory(factory.Factory):

    class Meta:
        model = Task

    title = factory.Sequence(lambda n: 'Title {}'.format(n))
    description = factory.Sequence(lambda d: 'Description {}'.format(d))
    cost = 5000.00
    assignee = factory.SubFactory(UserFactoryExecutor)
    created_by = factory.SubFactory(UserFactoryCustomer)

That is the example of my test

class ApiModelTestCase(TestCase):

     def test_creating_models_instance(self):
         executor = factories.UserFactoryExecutor()
         customer = factories.UserFactoryCustomer()
         Task.objects.create(title="Simple Task", description="Simple Description", cost="5000.00",
                            assignee=executor, created_by=customer)

This is how my mistake

ERROR: test_creating_models_instance (tests.test_models.ApiModelTestCase)

Traceback (most recent call last):
  File "/Users/heartprogrammer/Desktop/freelance-with-api/freelance/tests/test_models.py", line 12, in test_creating_models_instance
    assignee=executor, created_by=customer)
  File "/Users/heartprogrammer/Documents/envs/freelance/lib/python3.6/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/heartprogrammer/Documents/envs/freelance/lib/python3.6/site-packages/django/db/models/query.py", line 394, in create
    obj.save(force_insert=True, using=self.db)
  File "/Users/heartprogrammer/Documents/envs/freelance/lib/python3.6/site-packages/django/db/models/base.py", line 763, in save
    "unsaved related object '%s'." % field.name
ValueError: save() prohibited to prevent data loss due to unsaved related object 'assignee'.