过滤模型对象，然后选择一个ForeignKey字段

Question

I have these models in Django:

class Person(models.Model):
    ...

class Group(models.Model):
    ...

class Membership(models.Model):
    person = models.ForeignKey(Person, related_name='memberships')
    group = models.ForeignKey(Group, related_name='memberships')
    is_admin = models.BooleanField(default=False)

I want a Group method that returns the admins, as a list of Person instances.
These are the two solutions I found:

class Group(models.Model):
    ...

    @property
    def admins1(self):
        admin_ids = list(self.memberships.filter(is_admin=True).values_list('person', flat=True))
        return list(map(lambda ai: Person.objects.get(pk=ai), admin_ids))

    @property
    def admins2(self):
        admin_memberships = list(self.memberships.filter(is_admin=True))
        return list(map(lambda am: am.person, admin_memberships))

Do you have a better idea, eg a QuerySet method like values_list that keeps Person instances instead of person_ids?

Answer 1

The principle here is always that if you need Person instances, start from Person. Then it becomes simply a matter of following the relationships:

Person.objects.filter(membership__is_admin=True, membership__group=self)

Also note that Membership is the through table of a many-to-many relationship between Person and Group, so your queries become clearer if you explicitly define it as such:

class Person(models.Model):
    ...
    groups = models.ManyToManyField("Group", through="Membership")

Now the above query can be:

Person.objects.filter(membership__is_admin=True, groups=self)