sql - 使用 ActiveRecord 获取一系列有条件的计数

Question

我正在显示一个 Subscription 计数的图表，这些计数可能在某个时候被 soft_destroyed_at。

为此，我每个月都运行一次查询，这当然不如一个大查询好，但我的 SQL 技能又一次让我失望了。

这是我在 Ruby 中的做法:

months = (0..12).map { |i| i.months.ago.end_of_month }

stats = Hash[months.map do |eom|
  [
    eom.beginning_of_month.to_date,
    Subscription.where(
      'created_at < ?' \
      'AND (soft_destroyed_at IS NULL ' \
      '  OR soft_destroyed_at > ?) ' \
      'AND (suspended_at IS NULL ' \
      '  OR suspended_at > ?)',
      eom, eom, eom
    ).count
  ]
end]

# => { 2018-04-01 => 10, 2018-03-01 => 15, ... }

我如何使用 ActiveRecord 将其写成一个查询——或者在必要时使用原始 SQL>

数据库是 Postgres 10.2，应用程序是 Rails 5.2。

谢谢!

Answer 1

你可以使用这个查询(我在 2017 年使用了 12 个月；随意更改)。正如您在评论中所说，这假设一个 Postgresql 数据库:

query = 
  "select months.month, count(created_at) "\
  "from "\
    "(select DATE '2017-01-01'+(interval '1' month * generate_series(0,11)) as month, "\
            "DATE '2017-02-01'+(interval '1' month * generate_series(0,11)) as next) months "\
    "outer join subscriptions on "\
    "created_at < month and "\
    "(soft_destroyed_at IS NULL or soft_destroyed_at >= next) and "\
    "(suspended_at IS NULL OR suspended_at >= next) "\
  "group by month "\
  "order by month"

results = ActiveRecord::Base.connection.execute(query)

第一个子查询(from 中的 select)生成如下:

month                 next
"2017-01-01 00:00:00";"2017-02-01 00:00:00"
"2017-02-01 00:00:00";"2017-03-01 00:00:00"
"2017-03-01 00:00:00";"2017-04-01 00:00:00"
"2017-04-01 00:00:00";"2017-05-01 00:00:00"
"2017-05-01 00:00:00";"2017-06-01 00:00:00"
"2017-06-01 00:00:00";"2017-07-01 00:00:00"
"2017-07-01 00:00:00";"2017-08-01 00:00:00"
"2017-08-01 00:00:00";"2017-09-01 00:00:00"
"2017-09-01 00:00:00";"2017-10-01 00:00:00"
"2017-10-01 00:00:00";"2017-11-01 00:00:00"
"2017-11-01 00:00:00";"2017-12-01 00:00:00"
"2017-12-01 00:00:00";"2018-01-01 00:00:00"

Next 仅用于更轻松地检查订阅是否至少在下个月之前处于事件状态(销毁或暂停是 >= next(保证订阅者在当月处于事件状态)。