我有以下型号：

class Volunteer(models.Model):
    first_name = models.CharField(max_length=50L)
    last_name = models.CharField(max_length=50L)    
    email = models.CharField(max_length=50L)
    gender = models.CharField(max_length=1, choices=GENDER_CHOICES)


class Department(models.Model):
    name = models.CharField(max_length=50L, unique=True)
    overseer = models.ForeignKey(Volunteer, blank=True, null=True)
    location = models.CharField(max_length=100L, null=True)


class DepartmentVolunteer(models.Model):
    volunteer = models.ForeignKey(Volunteer)
    department = models.ForeignKey(Department)
    assistant = models.BooleanField(default=False)
    keyman = models.BooleanField(default=False)
    captain = models.BooleanField(default=False)
    location = models.CharField(max_length=100L, blank=True, null=True)

我想查询所有未分配志愿者的部门。我可以使用以下查询进行操作：

SELECT 
    vsp_department.name 
FROM   
    vsp_department 
LEFT JOIN vsp_departmentvolunteer ON vsp_department.id = vsp_departmentvolunteer.department_id  
WHERE
    vsp_departmentvolunteer.department_id IS NULL;

有没有一种更像django的方式做到这一点，还是我应该只使用原始sql？