ActiveRecord组和计数-count_id来自何处？ [英] ActiveRecord Group and Count - Where Does count_id Come From?

问题描述

我有 Order 和 Item 模型，还有 OrderItem ：

class OrderItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :item

此查询最受欢迎商品的ID：

This query finds the IDs of the most popular items:

OrderItem.group(:item_id).order("count_id DESC").count("id").first(10).map(&:first)

这有效，但什么是 count_id ？为什么这样做？ order（ count_id ... ）在幕后发生了什么？

This works, but what is count_id? Why does this work? What is going on here behind the scenes with order("count_id...?

推荐答案

count_id 是Rails赋予SQL聚合函数 COUNT（ order_items。 id）。

count_id is an alias that Rails gives to the SQL aggregate function COUNT("order_items"."id").

OrderItem.group（：item_id）.count（：id）计算行数对于每个单个项目：

OrderItem.group(:item_id).count(:id) counts the number of rows for each individual item:

SELECT COUNT("order_items"."id") AS count_id, item_id AS item_id FROM "order_items"  GROUP BY item_id

count_id 背后的魔力部分在 ActiveRecord中： ：计算 。别名在此处确定：

The magic behind the count_id part is in ActiveRecord::Calculations. The alias is determined here:

aggregate_alias = column_alias_for([operation, column_name].join(' '))

的方法注释column_alias_for 列表显示此示例：

#   column_alias_for("count", "id")              # => "count_id"

之后，订单列出项目出现的次数从大到小。返回值将是一个哈希值，其中键是项ID，值是出现次数。

After that, order lists the Item occurrences from largest to smallest. The return value will be a hash where the keys are Item IDs and the values are the count of occurrences.

第一抓取出现次数最多的10个项目，然后 map 提取 item_id 值。

first grabs the 10 items with the most occurrences, and map pulls out the item_id values.

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