django custom command complains AssertionError: ForeignKey(None) is invalid. why - django

Question

I've followed django custom command tutorial, the link is here.
My working directory looks like this:
myapps/
__init__.py
models.py
management/
__init__.py
commands/
__init__.py
my_command.py
tests.py
views.py
My code looks like this:
from django.core.management.base import BaseCommand, CommandError
class Command(BaseCommand):
def handle(self, *args, **options):
print '=========='
self.stdout.write('Successfully closed poll ttt')
When I run the command manage.py my_command, I got the following errors,
File "D:/ERP\apps\person\salary\models.py", line 8, in <module>
class Salarys(models.Model):
File "D:/ERP\apps\person\salary\models.py", line 14, in Salarys
Unit = models.ForeignKey(Units, verbose_name = u'def_unit, on_delete = models.PROTECT)
File "D:\Python27\Lib\site-packages\django\db\models\fields\related.py", line 910, in __init__
assert isinstance(to, basestring), "%s(%r) is invalid. First parameter to ForeignKey must be either a model, a model name, or the string %r" % (self.__class__.__name__, to, RECURSIVE_RELATIONSHIP_CONSTANT)
AssertionError: ForeignKey(None) is invalid. First parameter to ForeignKey must be either a model, a model name, or the string 'self'
Obviously, the first parameter of ForeignKey is my model Units, how do I silence the compiler's complaint?
ps: my model looks like this:
my model looks like this now.
class Salarys(models.Model):
'''
describe : salary table
author : liyang 2013-1-23
'''
User = models.ForeignKey(Users, verbose_name = u'account', on_delete = models.PROTECT)
Unit = models.ForeignKey(Units, verbose_name = u'def_unit', on_delete = models.PROTECT, null=True)
yy = models.IntegerField(u'year)
mm = models.IntegerField(u'month')
class Meta:
db_table = 'users_salarys'
class Units(models.Model):
'''
describe : def unit model
author : liyang 2012-12-4 11:45
'''
name = models.CharField(u'name',max_length = 20)
cname = models.CharField(u'company name',max_length = 20, blank = True, null = True)
aname = models.CharField(u'company short cut',max_length = 20, blank = True, null = True)
telephone = models.CharField(u'contact',max_length = 20, blank = True, null = True)
website = models.CharField(u'website',max_length = 25, blank = True, null = True)
address = models.CharField(u'address',max_length = 50, blank = True, null = True)
class Meta:
db_table = 'units'
....
the strange things are
1: User foreignkey does not make any troubles while Unit does...
2: my web server can be run without any problem while the command line can not be run...

Answer 1

Your class Units should come before your class Salarys:
class Units(models.Model):
...
class Salarys(models.Model):
user = models.ForeignKey(Users, verbose_name = u'account', on_delete = models.PROTECT)
unit = models.ForeignKey(Units, verbose_name = u'def_unit', on_delete = models.PROTECT, null=True)
One more recommendation: it's a best practice to name your model in singular. Django will automatically "pluralize" them. If Django fails to pluralize the class name properly, you can specify your own plural by adding the following to the models Meta:
class Meta:
verbose_name_plural = "salaries"