Django 只从查询集中获取某种类型的最新项目

两种型号:

class University(models.Model):
    name = models.CharField("name", max_length = 48)
    city = models.ForeignKey(City, on_delete = models.CASCADE) 

class Course(models.Model):
    name = models.CharField("course name", max_length = 48)
    university = models.ForeignKey(University, on_delete = models.CASCADE)
    students = models.ManyToManyField(Student, related_name = "%(class)s_name", related_query_name = "student_course_qs", blank = True)
    online = models.BooleanField("online course", default = False)
    semester = models.PositiveIntegerField("choose 1-6", default = 1, choices = [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)])
    time_update = models.DateTimeField("last info update", auto_now = True)

限制:如果Course有students，则不能是online，反之亦然.

如何获得给定city的所有Universities上的所有Courses的列表，其中Course上的latest个条目有students个，而Course个条目保存在第三个semester中？

我试着对University个不同的组进行循环，但这导致了大量的查询:

universities = University.objects.filter(city = City.objects.first())

wanted = []
for univ in universities:
    c = Course.objects.filter(university = univ).order_by("time_update").last()
    if c.semester == 3 and not c.online:
        wanted.append(c.id)

selected_courses = Courses.objects.filter(id__in = wanted)
    

我在这里看到的问题是，我创建了太多的查询，在筛选universities个查询之后，它们就已经分组了--我可以一个接一个地处理这些组，而不重新执行查询吗？