python - Python/Django Web 应用程序中数据库访问查询的性能优化

Question

长话短说

在我维护的 Django 应用程序中，我从 FOR 循环内部进行数据库调用。馊主意;我想把它带到循环之外。这是代码:

for link in context["object_list"]:
        try:
            latest_reply = link.publicreply_set.latest('submitted_on')
            #if latest_reply is something:
               #do something
        except:
            pass

FOR 循环外的数据库调用是什么？如您所见，我正在尝试获取每个 link 对象(外键关系)的 latest publicreply。请注意，并非每个 link 对象都存在 publicreply。我似乎无法全神贯注于如何在循环之外执行此操作。分析告诉我，这种重复调用会增加大量开销。

---

更多详情:

模型是:

class Link(models.Model):
    description = models.TextField(validators=[MaxLengthValidator(500)])
    submitter = models.ForeignKey(User)
    submitted_on = models.DateTimeField(auto_now_add=True)

class Publicreply(models.Model):
    submitted_by = models.ForeignKey(User)
    answer_to = models.ForeignKey(Link)
    submitted_on = models.DateTimeField(auto_now_add=True)
    description = models.TextField(validators=[MaxLengthValidator(250)])

class Seen(models.Model):
    seen_status = models.BooleanField(default=False)
    seen_user = models.ForeignKey(User)
    seen_at = models.DateTimeField(auto_now_add=True)
    which_reply = models.ForeignKey(Publicreply, related_name="publicreply_seen_related")

并在顶部的代码片段中添加更多附带代码:

link_ids = [link.id for link in context["object_list"]]
seen_replies = Publicreply.objects.filter(answer_to_id__in=link_ids,publicreply_seen_related__seen_user = user)
for link in context["object_list"]:
        try:
            latest_reply = link.publicreply_set.latest('submitted_on')
            if latest_reply in seen_replies:
               #do something
        except:
            pass

最后，context["object_list"] 是 link 对象的列表。对于 Django 模板中显示的每个 link 对象，如果它有一个 latest_reply，我会将它与某个时间戳进行比较，如果某些条件为真，我会放入视觉标记。

Answer 1

尝试:

latest_replys = Publicreply.objects.all().order_by('answer_to','-submitted_on').distinct('answer_to')

for reply in latest_replys:
    if reply in in seen_replies:
        #do something