Python 在Admin中显示从ManyToMany通过模型的筛选结果

这是我的模型.py:

class Person(models.Model):

    surname = models.CharField(max_length=100, blank=True, null=True)
    forename = models.CharField(max_length=100, blank=True, null=True)

    def __str__(self):
        return '{}, {}'.format(self.surname, self.forename)

class PersonRole(models.Model):
    ROLE_CHOICES = [
        ("Principal investigator", "Principal investigator"),
        [etc...]
    ]
    title = models.CharField(choices=TITLE_CHOICES, max_length=9)
    project = models.ForeignKey('Project', on_delete=models.CASCADE)
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    person_role = models.CharField(choices=ROLE_CHOICES, max_length=30)

    def __str__(self):
        return '{}: {} as {}.'.format(self.project, self.person, self.person_role)

class Project(models.Model):

    title = models.CharField(max_length=200)
    person = models.ManyToManyField(Person, through=PersonRole)

    def __str__(self):
        return self.title

    def get_PI(self, obj):
        return [p.person for p in self.person.all()] #I'll then need to filter where person_role is 'Principal investigator', which should be the easy bit.

在我的管理后台，我想显示的人(主要调查员)在主表:

class ProjectAdmin(ImportExportModelAdmin):

    list_filter = [PersonFilter, FunderFilter]
    list_display = ("title", "get_PI")
    ordering = ('title',)

我想要:在管理员的项目表中显示具有‘首席调查员’角色的人.

你可以看到我在我的models.py中创建了我的get_PI()，在我的list_display中创建了引用.我得到Project.get_PI() missing 1 required positional argument: 'obj'.我做错了什么？