Django之Mode的外键自关联和引用未定义的Model方法

class Department(models.Model):
 
  '''
  some other filed
  '''
  super_department = models.ForeignKey(Department)

class Department(models.Model):
  
  '''
  some other filed
  '''
  super_department = models.ForeignKey('self')

class Department(models.Model):
  
  name = models.CharField(max_length=20)
  leader = models.ForeignKey(User, related_name = 'lead_group', null = True)
 
class User(models.Model):
  
  username = models.CharField(max_length = 20)
  department = models.ForeignKey(Department, related_name = 'users')

class Department(models.Model):
  
  name = models.CharField(max_length=20)
  leader = models.ForeignKey('User', related_name = 'lead_group')#User now be str type
 
class User(models.Model):
  
  username = models.CharField(max_length = 20)
  department = models.ForeignKey(Department, related_name = 'users')