将计算列添加到ActiveRecord查询 [英] Adding a computed column to an ActiveRecord query

问题描述

我正在使用范围和某些条件来运行查询。像这样的东西：

I am running a query using a scope and some conditions. Something like this:

conditions[:offset] = (options[:page].to_i - 1) * PAGE_SIZE unless options[:page].blank?    
conditions[:limit] = options[:limit] ||= PAGE_SIZE
scope = Promo.enabled.active
results = scope.all conditions 

我想向查询中添加一个计算列（在我现在调用scope.all的时候）。像这样的东西：

I'd like to add a computed column to the query (at the point when I'm now calling scope.all). Something like this:

（ACOS（最小（1，COS（0.71106459055501）* COS（-1.2915436464758）* COS（RADIANS（addresses.lat）））* COS（RADIANS （addresses.lng））+
COS（0.71106459055501）* SIN（-1.2915436464758）* COS（RADIANS（addresses.lat））* SIN（RADIANS（addresses.lng））+
SIN（0.71106459055501） * SIN（RADIANS（addresses.lat）））* 3963.19）作为precision_distance

(ACOS(least(1,COS(0.71106459055501)*COS(-1.2915436464758)*COS(RADIANS(addresses.lat))*COS(RADIANS(addresses.lng))+ COS(0.71106459055501)*SIN(-1.2915436464758)*COS(RADIANS(addresses.lat))*SIN(RADIANS(addresses.lng))+ SIN(0.71106459055501)*SIN(RADIANS(addresses.lat))))*3963.19) as accurate_distance

有没有一种方法可以做到，而不仅仅是使用find_by_sql并重写整个现有查询？

Is there a way to do that without just using find_by_sql and rewriting the whole existing query?

谢谢！

推荐答案

当然，请使用：

conditions = Hash.new
conditions[:select] = "#{Promo.quoted_table_name}.*, (ACOS(...)) AS accurate_distance")
conditions[:offset] = (options[:page].to_i - 1) * PAGE_SIZE unless options[:page].blank?    
conditions[:limit] = options[:limit] ||= PAGE_SIZE
scope = Promo.enabled.active
results = scope.all conditions 

注意新的：select-告诉ActiveRecord您要返回的列。结果中返回的对象将具有#accurante_distance访问器。不幸的是，ActiveRecord太笨了，无法推断列的类型。您可以随时添加一个方法：

Note the new :select - that tells ActiveRecord what columns you want returned. The returned object in results will have a #accurante_distance accessor. Unfortunately, ActiveRecord is dumb and won't be able to infer the column's type. You can always add a method:

class Promo
  def accurate_distance
    raise "Missing attribute" unless has_attribute?(:accurate_distance)
    read_attribute(:accurate_distance).to_f # or instantiate a BigDecimal
  end
end

有关详细信息，请参见 #has_attribute 详细信息。

See #has_attribute for details.

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