Much like other ORMs, ActiveRecord makes it almost effortless to accidentally do very stupid queries in bulk. For example (from http://guides.rubyonrails.org/active_record_querying.html) this code

  clients = Client.all(:limit => 10)
  clients.each do |client|
      puts client.address.postcode
  end

looks harmless but requires eleven sequential round-trips to the database. This version

  clients = Client.includes(:address).limit(10)
  clients.each do |client|
      puts client.address.postcode
  end

still requires two, and one of the queries gets bigger as the number of ids increases. You can get into find_by_sql, but at that point ActiveRecord isn't adding any value.