Activerecord：提取特定的列和关联的关联计数 [英] Activerecord: Pluck specific columns and association's association counts

问题描述

好的，所以我们有个帐户，其中有许多时间表，其中有许多展示次数。

Alright, so we have Accounts which has many Schedules which has many Impressions.

我想获取 Accounts.name ，所有 Accounts.schedules ，但仅采摘 Accounts.schedules.date ， Accounts.schedules.summary 以及 Accounts.schedules.impressions

I want to get Accounts.name, all of Accounts.schedules, but only plucking Accounts.schedules.date, Accounts.schedules.summary and also the count of Accounts.schedules.impressions

这里是给定1个帐户的可视化视图（我想要所有帐户）

Here's a visualization of what I want given 1 account (I'll want all accounts)

Account:  
    .name
    Schedules
        .date
        .summary
        count(Schedule.impressions)

这是代码看起来好像查询太多了

And here's what the code would look like with too many queries

Account.find_each do |account|
    puts a.name # Account.name
    a.schedules.each do |schedule|
        puts schedule.date # Account.schedule.date
        puts schedule.summary # Account.schedule.summary
        puts schedule.impressions.count # Count(Account.schedule.impressions)

我想在尽可能少的查询中做到这一点，也许还学到一些东西！我喜欢效率和优化。

I'd like to do this in as few queries as possible, and maybe learn something too! I love efficiency and optimizations.

推荐答案

使用选择查询

Account.
  joins(schedules: :impressions).
  select("accounts.name as account_name, 
          schedules.date as schedule_date, 
          schedules.summary as schedule_summary, 
          count(impressions.id) as schedule_impression_count" ).
  group("accounts.name,schedules.date,schedules.summary") 

应该在单个查询中执行此操作，对象将响应列别名，如 schedule_date 和 schedule_impression_count

Should do it in a single query then the objects will respond the the column alias' like schedule_date and schedule_impression_count

如果您需要在同一视图中将这些对象用于其他数据，则 Account.eager_load（schedules：：impressions）也应工作，并且还将执行一个查询。

If you need to use these objects for other data in the same view then Account.eager_load(schedules: :impressions) should also work and will also execute a single query.

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