如何为 Django 中的 ManyToManyField 设置默认值

Question

edit- updated 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()
                 

My query/requirement: Employee and Department are linked by M2M. I set blank=True to overcome the form validation of no department being chosen. (null=True doesn't seem to be working with M2M)

Let's say,

scenario 1: while creating a new Emp, if there is no department provided then it should set to RP(Resource pool) by default.

scenario 2: An existing Emp is from the HR department; assuming for some reason the HR dept is being deleted or Emp is being kicked out, now that Emp has no department linked to it, it should move to RP(Resource pool) by default!!

How to arrange the default department- RP in Django such that both the scenarios are passing??

Thanks in advance:)

Dueces

Answer 1

As suggested here The right way to set the default value of a ManyToMany field in django , you have a few options.

Here is how to do so by overriding the save method:

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)

Here is how to do so using a post_save receiver:

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()