前提

• オブジェクトの配列をビューで表示したい
• objectはActiveRecordとは関係ない
• オブジェクトには一意の値が振られていない

renderをeachで回すとレンダリングが遅い

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- @records.each_with_index do |record, index|
  = render 'record', record: record, index: index

= "#{index}番目のrecord"

  Rendering records/index.html.slim within layouts/application
  Rendered records/_record.html.slim (7.8ms)
  Rendered records/_record.html.slim (0.2ms)
  Rendered records/_record.html.slim (0.7ms)
  Rendered records/_record.html.slim (0.2ms)
  Rendered records/_record.html.slim (0.3ms)
  Rendered records/_record.html.slim (0.8ms)
  Rendered records/_record.html.slim (0.3ms)
  Rendered records/_record.html.slim (0.2ms)
  Rendered records/_record.html.slim (0.3ms)
  Rendered records/_record.html.slim (0.4ms)
  Rendered records/_record.html.slim (0.7ms)
  Rendered records/_record.html.slim (0.3ms)
  Rendered records/_record.html.slim (0.2ms)
  Rendered records/_record.html.slim (0.2ms)
  Rendered records/_record.html.slim (0.2ms)
  Rendered records/_record.html.slim (0.3ms)
  Rendered records/index.html.slim within layouts/application (40.8ms)
Completed 200 OK in 1373ms (Views: 53.3ms | ActiveRecord: 0.0ms)

indexの代わりにrecord_counterを用いる

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= render partial: 'record', collection: @records

= "#{record_counter}番目のrecord"

このように、hoge_counterというメソッドで、それぞれ一意のindexが取得できる。

  Rendering records/index.html.slim within layouts/application
  Rendered collection of records/_record.html.slim [16 times] (132.2ms)
  Rendered records/index.html.slim within layouts/application (147.1ms)
Completed 200 OK in 1025ms (Views: 176.6ms | ActiveRecord: 0.0ms)

Railsには便利なメソッドが用意されているもんだ。