Activerecord: Pluck specific columns and association's association counts

问题

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

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

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

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)

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

回答1:

Use a select Query

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") 

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

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.

来源：https://stackoverflow.com/questions/37081020/activerecord-pluck-specific-columns-and-associations-association-counts