AttributeError：'ManyToManyField'对象没有属性'_m2m_reverse_name_cache' [英] AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'

问题描述

• 每个员工都可以是项目经理。

  >



• 项目经理可以有多个员工。




• 员工只能有1个项目经理。 p>

但是当我做 Employee.objects.get（name ='HereHere'）。get_xxx ）

我得到 AttributeError：'ManyToManyField'对象没有属性'_m2m_reverse_name_cache'

  class Employee（models.Model）：
 name = models.CharField（max_length = 20，unique = True）
 pm = models.ManyToManyField（'self'，symmetric = False，through ='PM'，related_name ='related_to'，）
 
 def add_pm ，employee，）
 pm，created = PM.objects.get_or_create（from_employee = self，to_employee__in = employee）
 return pm 
 
 def remove_pm（self，employee，） ：
 PM.objects.filter（
 from_employee = self，
 to_employee = employee，
）.delete（）
 return 
 
 def get_relationships（self，） ：
 return self.pm.filter（
 to_employee__from_employee = self）＃< ----- AttributeError：'ManyToManyField'对象没有属性'_m2m_reverse_name_cache'
 
 def get_related_to （self，）：
 return self.related_to.filter（
 from_employee__to_employee = self）＃< ----- AttributeError：'ManyToManyField'对象没有属性'_m2m_reverse_name_cache'
 
 
 def __str __（self）：
 return'％s'％self.name 
 
 
 class PM（models.Model）：
 from_employee = models.ForeignKey（Employee，related_name ='from_employee'）
 to_employee = models.ManyToManyField（Employee，related_name ='to_employee'）＃< -----这导致问题
  

解我做了一个你上面代码的应用程序，并设法重新创建了这个问题。

我尝试将'self'切换成'Employee'为建议此处，并尝试调整其他一些事情（像 on_delete = models.CASCADE ），但仍然实例化一个Employee对象，并调用 .pm 这个错误。

我认为django对于您可以通过通过参数为ManyToMany而使用什么类别的期望，以及它必须有两个外键，而不是一个外键和一个ManyToMany。

所以...

如果你切换到这个：

  class PM（models.Model）：
 from_employee = models.ForeignKey（Employee， related_name ='from_employee'）
 to_employee = models.ForeignKey（Employee，related_name ='to_employee'）
  

它的作品。这实际上是ManyToMany关系的正常模式 - 每个PM代表项目经理关系，而不是一个人。

或者， h2>

您可以让项目经理成为从员工到员工的外键，命名为 managed_by ，以确保每位员工能只有一个项目经理。

I'm trying to assign Project Manager to Employee.

• Every Employee, can be Project Manager.

• Project Managers can have multiple Employees.

• Employees can have only 1 Project Manager.

But When I do Employee.objects.get(name='HereHere').get_xxx()

I got AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'

class Employee(models.Model):
    name = models.CharField(max_length=20, unique=True)
    pm = models.ManyToManyField('self', symmetrical=False, through='PM', related_name='related_to', )

    def add_pm(self, employee, ):
        pm, created = PM.objects.get_or_create(from_employee=self, to_employee__in=employee,)
        return pm

    def remove_pm(self, employee, ):
        PM.objects.filter(
            from_employee=self,
            to_employee=employee,
        ).delete()
        return

    def get_relationships(self, ):
        return self.pm.filter(
            to_employee__from_employee=self)   #<----- AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'

    def get_related_to(self,):
        return self.related_to.filter(
            from_employee__to_employee=self)    #<----- AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'


    def __str__(self):
        return '%s' % self.name


class PM(models.Model):
    from_employee = models.ForeignKey(Employee, related_name='from_employee')
    to_employee = models.ManyToManyField(Employee, related_name='to_employee') #<----- This cause the problem

解决方案

I made an app with your code above and managed to recreate the issue.

I tried switching 'self' to 'Employee' as suggested here and tried tweaking a couple other things (like on_delete=models.CASCADE) in the field, but still instantiating an Employee object and calling .pm on it threw the error.

I think django has expectations about what classes you can use as the through parameter for a ManyToMany and it has to have two foreign keys, not a foreign key and a ManyToMany.

So...

If you switch to this:

class PM(models.Model):
    from_employee = models.ForeignKey(Employee, related_name='from_employee')
    to_employee = models.ForeignKey(Employee, related_name='to_employee')

it works. And that's actually the normal pattern for ManyToMany relationships anyways -- each PM represents a Project Manager relationship, not a person.

Alternatively,

You could have project manager be a foreign key from Employee to Employee, named something like managed_by to make sure each employee can only have one project manager.

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