后保存信号和关系

Question

我正在应用post_save信号来为每个对象应用用户权限，然后相应地过滤查询集。

我的模型是这样的：

class Project(models.Model):

    # Relations with other entities.

    employees = models.ManyToManyField('staff.Person', through='project.PersonProjectMembership',

                                       related_name='projects')

    research_groups = models.ManyToManyField('group.Group', related_name='projects',

                                             through='project.ProjectGroupMembership')

    departments = models.ManyToManyField('department.Department', related_name='projects',

                                         through='project.ProjectDepartmentMembership')

问题是：当我收到保存后的信号时，尽管我已经输入了部门，research_groups和员工的值，但它们似乎总是空着的。有什么我想念的吗？

更新：在当前代码下方，该代码尚未按预期运行。我已将post_save更改为m2m_changed。

from django.db.models.signals import m2m_changed

from django.db import models

from django.dispatch.dispatcher import receiver

class Project(models.Model):

    employees = models.ManyToManyField('staff.Person', through='project.PersonProjectMembership',

    related_name='projects')

class PersonProjectMembership(models.Model):

    project = models.ForeignKey('project.Project', related_name="person_memberships")

    person = models.ForeignKey('staff.Person', related_name="project_memberships")

    lead = models.BooleanField(default=False)

    position = models.CharField(max_length=50)

    project_manager = models.BooleanField(

        default=False

    )

    class Meta:

        permissions = (

            ('view_personprojectmembership', _('View person project membership')),

        )

@receiver(m2m_changed, sender=Project.employees.through)

def _on_save_project_assign_privileges(sender, instance, action, reverse, model, pk_set, using, **kwargs):

    # [...]

Answer 1

您可能知道，Many2Many关系是通过附加表存储的，该表包含关系两端的主键。因此，与Many2Many关系的保存模型实例是两个步骤的过程：首先，实例被保存：数据库中的新记录被创建并且实例接收它的主键。post_save此时被解雇。之后，关系被保存：在关系表中创建记录。m2m-changed此时会触发信号。换句话说，当post_save被解雇时，尚未处理m2m关系。

Answer 2

在深入研究代码并进行简单测试之后，我尝试了krasnoperov建议的m2m_changed信号。我意识到，如果您明确声明一个“直通”模型（在本例中为“ PersonProjectMembership”），则此信号将无法正常工作。然后我再次考虑，并将我的方法链接到“ PersonProjectMembership”的post_save信号。那很好。