取最小/最大的每个组中的ActiveRecord [英] Fetching Minimum/Maximum for each group in ActiveRecord

问题描述

这是给定一个表带有属性的类型，品种和价格一个古老的问题，你取纪录的最低价格为每一种存在。

在SQL中，我们可以做的这个通过：

 选择f.type，f.variety，f.price
从（选择类型，分（价格）为minprice从表中按类型）为x
内部连接表为f上f.type = x.type和f.price = x.minprice;`
 

我们也许可以模仿这种方式：

  minprices = Table.minimum（：价格，：组=＆GT;类型）
结果= []
minprices.each_pair做| T，P |
   结果＆LT;＆LT; Table.find（：第一，：条件=＆GT; [？类型=价=，T，P]）
结束
 

有没有更好的实现比这个？

解决方案

  Table.minimum（：价格，：组=＆GT;：型）
 

请参阅<一href="http://api.rubyonrails.org/classes/ActiveRecord/Calculations/ClassMethods.html">http://api.rubyonrails.org/classes/ActiveRecord/Calculations/ClassMethods.html更多。

This is an age-old question where given a table with attributes 'type', 'variety' and 'price', that you fetch the record with the minimum price for each type there is.

In SQL, we can do this by:

select f.type, f.variety, f.price   
from (  select type, min(price) as minprice from table group by type ) as x  
inner join table as f on f.type = x.type and f.price = x.minprice;`

We could perhaps imitate this by:

minprices = Table.minimum(:price, :group => type)  
result = []
minprices.each_pair do |t, p|  
   result << Table.find(:first, :conditions => ["type = ? and price = ?", t, p])
end

Is there a better implementation than this?

解决方案

Table.minimum(:price, :group => :type)

See http://api.rubyonrails.org/classes/ActiveRecord/Calculations/ClassMethods.html for more.

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