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把一组对象捏成一个对象

 3 years ago
source link: https://mednoter.com/group-active-record-querying.html
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把一组对象捏成一个对象

在电商系统中，经常要把一组对象加减乘除，生成报表。怎么处理这种需求呢？

方案1：按照统计要求创建一个model，将计算好的数据保存在里面。

方案2：使用group方法，把一组对象捏合为一个对象。

今天我说一下使用第二种方案的感受。

Active Record Query Interface: Group

这是 Group 方法的官方文档[1]。

To apply a GROUP BY clause to the SQL fired by the finder, you can specify the group method on the find. For example, if you want to find a collection of the dates orders were created on:

Order.select("date(created_at) as ordered_date, 
              sum(price) as total_price").group("date(created_at)”)

** And this will give you a single Order object for each date where there are orders in the database. ** The SQL that would be executed would be something like this:

SELECT date(created_at) as ordered_date, sum(price) as total_price
FROM orders
GROUP BY date(created_at)

使用 Group 可以做些什么呢？为了让你更好的呃理解，我讲一下我的应用case。

这个表创建的意义：记录每个商品每天的销量，销售总额

  • the_date: 日期
  • goods_id: 商品id
  • quantity: 该商品在当日的销售数量
  • amount: 该商品在当日的销售额度

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应用1：如何统计某个时间段内的热销商品？

# controller
def best_selling
  @stat = GoodsStat.best_selling(params[:start_date], params[:end_date])
end

# model
def self.best_selling(start_date, end_date)
  start_date = start_date.to_date
  end_date  = end_date.to_date

  where("the_date >= ? and the_date <= ?", start_date, end_date).
    group("goods_id").
    select("goods_id,
          sum(quantity) as quantity,
          sum(amount) as amount).order("quantity DESC").all
end

根据 goods_id 把一组数据「捏为」一个对象，这个对象的属性：

  • quantity：某商品在时间段内的销售总量
  • amount：某商品在时间段内的销售总额

应用2：统计每天的销售总额

#controller
def daily
  @stat = GoodsStat.best_selling(params[:start_date], params[:end_date])
end

#model
def self.best_selling(start_date, end_date)
  start_date = start_date.to_date
  end_date   = end_date.to_date

  where("the_date >= ? and the_date <= ?", start_date, end_date).
    group("the_date").
    select("goods_id,
            sum(quantity) as quantity,
            sum(amount) as amount).order("quantity DESC").all
end

根据 the_date 把某一天的所有对象「捏为」一个对象，这个对象的属性：

  • quantity：某一天销售的商品总数量
  • amount：某一天销售的商品总额度
  1. Rails Guide: Group, Active Record Query Interface
吕小荣
18 January 2014

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