Rails 嵌套 SQL 查询 [英] Rails Nested SQL Queries

问题描述

我有一个数据库模型 Position(lat,lon) 包含 latitudes 和 longitudes.

I have a database model Position(lat,lon) which holds latitudes and longitudes.

我有一个名为 show_close_by 的控制器操作，它接收以度为单位的位置 (my_lat, my_lon)、容差(以公里为单位)，并应返回位置列表公差范围内的数据库.

I have a controller action called show_close_by which recieves a position in degrees (my_lat, my_lon), a tolerance (in kilometers) and should return the list of positions in the database that are within tolerance range.

为此，我使用 hasrsine_distance 公式计算两个坐标之间的距离(在地球表面上)(lat1, lon1, lat2, lon2).

For that, I use the haversine_distance formula which calculates the distance in kilometers (on the surface of the Earth) between two coordinates (lat1, lon1, lat2, lon2).

为了使查询更快，我在查询中编写了整个 haversine_distance 公式:

To make the query faster, I wrote the whole haversine_distance formula in the query:

... WHERE 2*6371*asin(sqrt( power( sin( (:lat2-latitude)*pi()/(180*2) ) ,2) + cos(latitude*pi()/180)*cos(:lat2*pi()/180)*power(sin( (:lon2-longitude)*pi()/(180*2) ),2) )) < tolerance

查询的细节无关紧要.我的疑问是:是否有必要为数据库中的每个位置计算这个巨大的函数?可以用更简单的函数过滤掉一些明显太远的位置吗?

The specifics of the query don't matter. My doubt is: is it necessary to calculate this huge function for EVERY position in the database? Can I filter out some positions that are clearly too far away with a simpler function?

好吧，我可以:使用嵌套的 SQL 查询，我可以查询数据库中大正方形"(在纬度/经度空间)内的位置，然后使用成本更高的三角函数过滤那些位置.类似于以下内容:

Well, I can: With a nested SQL query, I can query the database for positions that are within a large "square" (in lat/lon space), and then filter those with the more costly trigonometric function. Something like the following:

SELECT * FROM ( SELECT * FROM Positions WHERE lat-latitude < some_reasonable_upper_bound AND lon-longitude < same_upper_bound ) WHERE costly_haversine_distance < tolerance

最后，我的问题是:如何在 Rails 中实现这一点(无需自己编写整个查询)?Positions.where(reasonable_upper_bound).where(costly_but_accurate_restriction) 是否进行嵌套查询?如果没有，怎么办?

Finally, my question: how can I implement this in Rails (without writing the whole query myself)? Does Positions.where(reasonable_upper_bound).where(costly_but_accurate_restriction) make a nested query? If not, how?

非常感谢！

推荐答案

以下是进行嵌套查询的方法:

Here's how to make nested queries:

LineItem.where(product_id: Product.where(price: 50))

它发出以下请求:

SELECT "line_items".* FROM "line_items" 
WHERE "line_items"."product_id" IN 
(SELECT "products"."id" FROM "products" WHERE "products"."price" = 50)

请注意，仅 id 将从 products 表中获取.如果你想用另一种方式连接两个实体而这种魔法不合适，请使用 Product.select(:some_field).where(...).

Note that only ids will be fetched from the products table. If you are trying to make another way to connect two entities and this magic isn't suitable, use Product.select(:some_field).where(...).

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