Django的具有“ not in”过滤器的巨型列表性能？ [英] Django performance with `not in` filter with giant list?

问题描述

我目前正在做一个学校项目，正在建立一个网站，让您对附近的餐馆（1-5星）进行评分。唯一的警告是，我将根据位置向用户显示餐厅（例如，最靠近的餐厅），并且一旦他们给餐厅评分，就永远不会再向他们显示。

I am currently making a school project where I am building a website that lets you rate nearby restaurants (1-5 stars). The only caveat is that I will show users the restaurants based on location (e.g. closest ones first) and that once they have rated a restaurant, it will never be shown to them again.

当前我有这样的想法

class User(models.Model):
    username = models.CharField(primary_key=True, max_length=20)
    location = models.SomeGeoPointField
    ...

class Restaurant(models.Model):
    title = models.CharField(primary_key=True, max_length=20)
    location = models.SomeGeoPointField
    ...

class Rating(models.Model):
    for    = models.ForeignKey(User, on_delete=models.CASCADE)
    from   = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
    rating = models.IntegerField()

我在思考查询的外观时遇到了麻烦，如果我不在其中使用 过滤掉现有评级，关于可能的市盈率

I am having trouble thinking how the query would look like, and if I used a not in to filter out existing ratings, about the possible performance hit

推荐答案

您需要的查询是一个排除用户已评价的餐厅的查询。如果执行此操作，Django将创建一个有效的查询：

The query you need is one that excludes the restaurants which the user has already rated. Django will create an efficient query if you do this:

Restaurant.objects.exclude(rating__user=request.user)

如果您意识到这里的内容是多对多的关系，则可以使查询变得更加容易用户和餐厅，以及评级直通表。因此：

You can make the queries even easier if you recognise that what you have here is a many-to-many relationship between User and Restaurant, with a through table of Rating. So:

class User(models.Model):
    ...
    ratings = models.ManyToManyField('Restaurant', through='Rating', related_name='users_rating')

实际上实际上改变了这个查询-现在是：

This doesn't actually alter this query very much - now it is:

Restaurant.objects.exclude(users_rating=request.user)

但它肯定会使其他查询更容易。

but it will certainly make other queries easier.

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