ActiveRecord的聚合函数：有一个数据库无关的“EXTRACT（周末从日）”？ [英] ActiveRecord aggregate function: is there a database-agnostic 'EXTRACT(WEEK from date)'?

问题描述

我有一个工作得很好（有轨3和PostgreSQL）方法;我只是想知道，如果有可能，使之与数据库无关的：

i have a method that works just fine (with rails 3 and PostgreSQL) ; i just wonder if it's possible to make it database-agnostic :

FarmGatePrice.average :price, :group => 'EXTRACT(WEEK FROM date)'

据我了解，方法提取的日期，每周ISO数值总是特定数据库。

As i understand it, methods to extract a week ISO number from a date are always database-specific.

我可以使用类似虚拟属性：本周看到的此处，但似乎：组选项只接受原始的SQL。我也看到这个解决方案，但它不适合我的需要。 另一种方法是使用一个数据库视图，甚​​至添加：每周一栏中填用回调方法 - 但我想preFER不要惹我的模式

I could use something like a virtual attribute :week as seen here, but it seems the :group options only accepts raw SQL. I've also seen this solution but it doesn't fit my needs. Another way would be to use a database view, or even to add a :week column filled by a callback method - but i'd prefer not to mess with my schema.

那么，什么线索？

推荐答案

都能跟得上 - 如果你需要每周做一组，你最好直接在数据库中保存为一个额外的列通过设置 before_save 回调。

Nope - if you need to do a group by week, you're better off saving it directly in the database as an extra column set via a before_save callback.

正如你所说的，每周从日期提取是不平凡的，所以最好还是让Rails的处理这个问题，而不是你的数据库。这将提高性能了。

As you say, extracting a week from a date is non-trivial, and so it's best to let Rails handle this rather than your database. This will improve performance too.

更新：

例回调：

def Thing

  before_save :set week

  private
     def set_week
        self.week = self.date.cweek
     end
end

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