Why is the associated emails collection of the mailbox saved in 3.1, but not in 3.0? (The underlying code is exactly the same other than the Rails version.)

save_associated console output

Rails 3.1:

>> Mailbox.first.emails.each(&:destroy); mb = Mailbox.first; eml = mb.emails.new(site: Site.first); mb.save

  Mailbox Load (0.6ms)  SELECT `mailboxes`.* FROM `mailboxes` WHERE `mailboxes`.`deleted_at` IS NULL LIMIT 1
  Email Load (0.3ms)  SELECT `emails`.* FROM `emails` WHERE `emails`.`mailbox_id` = 1
  Mailbox Load (0.3ms)  SELECT `mailboxes`.* FROM `mailboxes` WHERE `mailboxes`.`deleted_at` IS NULL LIMIT 1
  Site Load (0.4ms)  SELECT `sites`.* FROM `sites` WHERE `sites`.`deleted_at` IS NULL LIMIT 1
   (0.2ms)  BEGIN
   (0.4ms)  SELECT 1 FROM `mailboxes` WHERE (LOWER(`mailboxes`.`email`) = LOWER('[email protected]') AND `mailboxes`.`id` != 1 AND `mailboxes`.`site_id` = 3 AND `mailboxes`.`deleted_at` IS NULL) LIMIT 1
   (0.3ms)  UPDATE `mailboxes` SET `updated_at` = '2013-01-23 22:22:55', `comm_options` = '--- \n', `lock_version` = 5 WHERE (`mailboxes`.`id` = 1 AND `mailboxes`.`lock_version` = 4)
  SQL (0.4ms)  INSERT INTO `emails` (`bcc`, `body`, `cc`, `created_at`, `delivered_to`, `email_client_id`, `email_out_status_id`, `email_template_id`, `error_count`, `forward_from`, `forward_reply_to`, `from`, `header`, `hidden_at`, `hidden_by_id`, `in_reply_to`, `interaction_direction_id`, `last_error`, `mailbox_id`, `message_id`, `orig_email_id`, `orig_ticket_id`, `outbound_mailbox_id`, `reply_to`, `send_at`, `sender`, `sender_group_id`, `sender_user_id`, `sent_at`, `site_id`, `size`, `subject`, `ticket_attachments_count`, `ticket_id`, `to`, `uid`, `updated_at`, `x_mailer`) VALUES (NULL, NULL, NULL, '2013-01-23 22:22:55', NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, 1, NULL, 1, NULL, NULL, NULL, NULL, NULL, '2013-01-23 22:22:55', NULL, NULL, NULL, NULL, 2, NULL, '', 0, NULL, NULL, NULL, '2013-01-23 22:22:55', NULL)
   (0.4ms)  COMMIT


Rails 3.0:

>> Mailbox.first.emails.each(&:destroy); mb = Mailbox.first; eml = mb.emails.new(site: Site.first); mb.save

[DEBUG: 01/23 17:25:07 #32136]   Mailbox Load (0.4ms)  SELECT `mailboxes`.* FROM `mailboxes` WHERE `mailboxes`.`deleted_at` IS NULL LIMIT 1
[DEBUG: 01/23 17:25:07 #32136]   Site Load (0.3ms)  SELECT `sites`.* FROM `sites` WHERE `sites`.`deleted_at` IS NULL LIMIT 1
[DEBUG: 01/23 17:25:07 #32136]   SQL (0.4ms)  BEGIN
[DEBUG: 01/23 17:25:07 #32136]   SQL (0.4ms)  SELECT 1 FROM `mailboxes` WHERE `mailboxes`.`site_id` = 3 AND `mailboxes`.`deleted_at` IS NULL AND (LOWER(`mailboxes`.`email`) = LOWER('[email protected]')) AND (`mailboxes`.id <> 1) LIMIT 1
[DEBUG: 01/23 17:25:07 #32136]   AREL (0.4ms)  UPDATE `mailboxes` SET `comm_options` = NULL, `lock_version` = 6 WHERE (`mailboxes`.`id` = 1 AND `mailboxes`.`lock_version` = 5)
[DEBUG: 01/23 17:25:07 #32136]   SQL (0.5ms)  COMMIT