既存の提出物にデータを挿入するdata_migrationスクリプトを追加

db/data/20210816082555_add_last_comment_at.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class AddLastCommentAt < ActiveRecord::Migration[6.1]
+  def up
+    products = Product.where(self_last_comment_at: nil).where(mentor_last_comment_at: nil)
+    products.each do |product|
+      next unless product.comments.size.positive?
+
+      product.comments.each do |comment|
+        if comment.user.mentor
+          product.mentor_last_comment_at = comment.updated_at
+        elsif comment.user == product.user
+          product.self_last_comment_at = comment.updated_at
+        end
+      end
+      product.save!
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end