django模型中的m2m 'through‘字段抛出此错误：'M2M m2m’对象没有属性'_m2m_reverse_name_cache‘

Question

嘿，伙计们，我正在尝试添加一个m2m through字段，以便在我的“部门”模型中有像department.assistants.all()一样的呼叫助手，但在这样做的过程中，我得到了这个错误AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'。

这是我的模型：

class Department(models.Model):
    id                  = models.BigAutoField(primary_key=True)
    user                = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    assistants          = models.ManyToManyField(settings.AUTH_USER_MODEL, through='Assistants', related_name='dep_assistants', 
                            symmetrical=False)

class Assistants(models.Model):
    id                  = models.BigAutoField(primary_key=True)
    department          = models.ForeignKey(Department, related_name='of_department', on_delete=models.CASCADE)
    assistant           = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='dt_assistant', 
                            verbose_name="Department Assistant", on_delete=models.CASCADE)
    added               = models.DateTimeField(auto_now_add=True)

我对这个概念还很陌生。有人能告诉我我哪里做错了吗？

谢谢

Answer 1

按照你定义模型的方式，查询似乎太混乱了。尝试下面定义模型的方式，然后尝试查询。

您在多对多字段定义中没有提到through_field属性。查看文档：https://docs.djangoproject.com/en/3.2/ref/models/fields/#django.db.models.ManyToManyField

class Department(models.Model):
    # i think this is not needed. Also id is a protected keyword in python.
    # id                  = models.BigAutoField(primary_key=True)
    user                = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    assistants          = models.ManyToManyField(settings.AUTH_USER_MODEL, through='Assistants', 
                                                 related_name='departments', through_fields=("department", "assistant"))

# model name should never be prural. It is singluar becuase it is the name of the object.
class Assistant(models.Model):
    # i think this is not needed. Also id is a protected keyword in python.
    # id                  = models.BigAutoField(primary_key=True)
    department          = models.ForeignKey(Department, on_delete=models.CASCADE)
    assistant           = models.ForeignKey(settings.AUTH_USER_MODEL, verbose_name="Department Assistant", on_delete=models.CASCADE)
    added               = models.DateTimeField(auto_now_add=True)


# how to query assistants from departments
# you will get objects of User model
qs = department.assistants.all()


# how to query departments from assistants
# you will get objects of Department model
qs = user.departments.all()


# If you want to query the Assistant model

# from department object
qs = department.assistant_set.all()

# from assistant object
qs = user.assistant_set.all()

# in either case you will get the objects of Assistant model
for i in qs:
    print(i.added, i.department, i.assistant)

试一试，如果你仍然得到这个错误，请告诉我。

我的建议是将Assistant模型上的assistant字段命名为user。这样，您就不需要在多对多字段上定义through_field。

Answer 2

如果一个助理只与一个部门相关-这是一对多关系。(一个部门有许多助手)在代码中应该是：

class Assistant(models.Model):
    ...
    department = models.ForeignKey(Department)

不需要在部门上有特殊的参考。要获取所有助理：

assistants = models.Assistant.objects.filter(department=department)

或者在类Department上创建一个属性：

@property
def assistants(self):
    return models.Assistant.objects.filter(department=self)

如果一个助理涉及多个部门(每个部门有多个助理)，则是多对多关系，它们之间应该有额外的类：

class Assignment(models.Model):
    assistant = models.ForeignKey(Assistant)
    department = models.ForeignKey(Department)

class Department(models.Model):
    ...
    assignment= models.ForeignKey(Assignment)

class Assistant(models.Model):
    ...
    assignment = models.ForeignKey(Assignment)

因此，在这里查询部门的助理：

assistants = models.Assistant.objects.filter(
    assignment__in=models.Assignment.objects.filter(
        department=department
    )
)