如何在 Django 中为 ManyToManyField 设置默认值

Question

编辑-更新的models.py models.py：

from django.db import models
from django.db.models.signals import post_save
from django.dispatch import receiver

class Department(models.Model):
    departmentName = models.CharField(max_length=100)

    def __str__(self):
        return self.departmentName

class Designation(models.Model):
    designationName = models.CharField(max_length=100, null=True)

    def __str__(self):
        return self.designationName
 
class Employee(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField(unique=True, blank=False)
    department = models.ManyToManyField(Department, blank=True)
    designation = models.ForeignKey(Designation, null=True, on_delete=models.SET_NULL)

    def __str__(self):
        return self.name
    
    def departments(self):
        return ",".join([d.departmentName for d in self.department.all()])

#Using post receiver signal as suggested by @Daniel in answer and comments

@receiver(post_save, sender=Employee, dispatch_uid='set_department')
def SetDefaultDeparment(**kwargs):

    employee = kwargs['instance']
       
    if not employee.department.all(): 

        resource_pool = Department.objects.get(departmentName="RP")

        employee.department.add(resource_pool)
        employee.save()
                 

我的查询/要求： 员工和部门通过 M2M 链接。我设置 blank=True 来克服没有选择部门的表单验证。 （null=True 似乎不适用于 M2M）

比方说，

场景一：在新建 Emp 时，如果没有提供部门，则默认设置为 RP(Resource pool)。

场景 2： 现有 Emp 来自 HR 部门；假设由于某种原因 HR 部门被删除或 Emp 被踢出，现在 Emp 没有与之关联的部门，它应该默认移动到 RP（资源池）！

如何在 Django 中安排默认的部门-RP 以使两个场景都通过！？

提前致谢:)

应酬

Answer 1

正如此处The right way to set the default value of a ManyToMany field in django 所建议的那样，您有几个选择。

下面是如何通过覆盖保存方法来做到这一点：

class Employee(models.Model):

    ...
    
    def save(self): 

        # set department if field is empty:        
        if not self.department.all(): 

            # get the resource pool department:
            resource_pool = Department.objects.get(name='Resource Pool')

            # add to instance:
            self.department.add(resource_pool)
        
        super(Employee, self).save(*args, **kwargs)

以下是使用 post_save 接收器的方法：

from django.db.models.signals import post_save
from django.dispatch import receiver

# this decorator will execute the function after an Employee is saved:
@receiver(post_save, sender=Employee, dispatch_uid='set_department')
def SetDefaultDeparment(**kwargs):
    
    ''' sets a default department for an Employee where needed '''
    
    # unpack kwargs:
    employee = kwargs['instance']

    # set department if field is empty:        
    if not employee.department.all(): 

         # get the resource pool department:
         resource_pool = Department.objects.get(name='Resource Pool')

         # add to instance:
         employee.department.add(resource_pool)
         employee.save()