计算总的运行轨道3 [英] Calculate running total rails 3

问题描述

什么是最好的方式 - 或者更确切地说，任何方式 - 来计算总运行Rails中？

What's the best way - or, indeed, any way - to calculate a running total in Rails?

我有一个模型，销售。它有一个量列和sales_value列。我需要填充第三列，total_quantity，随着previous记录的数量值，当表由isbn_id分类的总和，那么CHANNEL_ID，然后invoice_date。这台各种合理的数据库管理敲响了警钟，所以我想知道如果它甚至有可能。

I have a model, Sale. It has a quantity column and a sales_value column. I need to populate a third column, total_quantity, with the sum of the quantity values of the previous records, when the table is sorted by isbn_id, then channel_id, then invoice_date. This sets all sorts of sensible database management alarm bells ringing, so I'm wondering if it's even possible.

的原因需要这个累计总和是一个百分比应用于销售，其中的累积量是在特定范围内。我不能在所有的记录，使用平均销售值，因为销售保证金可随时间变化很大 - 所以我的平均应用到了一堆销售可能高于或低于工资提成收款人。

The reason for needing this cumulative sum is to apply a percentage to the sales where the cumulative quantity is within a particular range. I can't use an average sales value across all records, because the margin on sales can vary dramatically over time - so I'd apply an average to a bunch of sales which might over or under pay the royalty payee.

所以。我应该做销售模型before_save回调，并update_attribute，不知何故？是否有返回表格时以特定方式排序的previous记录的值的方法？或者我应该转储全部销售记录到一个数组，也许还可以利用注入积累运行总？

So. Should I do a before_save callback on the Sale model, and update_attribute, somehow? Is there a method to return the value of the previous record when the table is sorted in a particular way? Or should I dump all Sale records into an array and maybe use inject to accumulate the running total?

任何想法非常受欢迎，在此先感谢。

Any ideas most welcome, thanks in advance.

更新：后续问题问这里。

推荐答案

不要使用注射（srsly）。要做到这一点，最好的方法是使用SQL命令组和/或计算中的ActiveRecord（如总和）方法

Do not use inject (srsly). The best way to do this is to use the SQL group commands and/or the Calculations methods in activerecord (like sum)

http://ar.rubyonrails.org/classes/ActiveRecord/Calculations/ ClassMethods.html

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