Django抱怨在子查询中使用查询时,必须在子查询中使用查询

我有以下习惯 QuerySet Program :

def with_volunteer_stats(self):
    from programs.models import Session
    volunteers_needed = ExpressionWrapper(
        F('num_student_seats') / F('student_volunteer_ratio'),
        output_field=models.IntegerField())

    sessions = (
        Session.objects.filter(program_id=OuterRef('pk'))
        .annotate(num_volunteers=Count('volunteer_attendances__volunteer', distinct=True))
        .order_by('num_volunteers')
    )
    qs = self.annotate(
        volunteers_needed=volunteers_needed,
        least_volunteers=Subquery(sessions.values('num_volunteers')[0])
    ).annotate(
        remaining_volunteers_needed=(F('volunteers_needed') - F('num_volunteers'))
    )
    return qs

运行此命令时,出现异常:

ValueError: This queryset contains a reference to an outer query and may only be used in a subquery.

回溯显示使用 Subquery . 这与Django文档中关于如何使用 . 知道我做错了什么吗?