ensl.org/db/migrate/20200402000314_move_readings_to_read_marks.rb

class MoveReadingsToReadMarks < ActiveRecord::Migration[6.0]
  def change
    execute "TRUNCATE read_marks"
    execute "INSERT INTO read_marks (readable_type, readable_id, reader_type, reader_id, timestamp)
             SELECT readable_type, readable_id, 'User', user_id, updated_at
             FROM readings GROUP BY readable_type, readable_id, user_id;"
  end
end