In Relation#empty? use #exists? instead of #count.
Merge pull request #12533 from szimek/use_exists_in_relation_empty

In Relation#empty? use #exists? instead of #count.
Remove extra variable creation and merge.
Instrument the generation of Action Mailer messages

The processing of outbound mail is instrumented with the key
`process.action_mailer`. The payload includes the mailer name as well as
the mailer method.
Merge pull request #12556 from dasch/dasch/nstrument-am-processing

Instrument the generation of Action Mailer messages
Merge pull request #12591 from vipulnsward/remove_mail_merge

Remove extra variable creation and merge.
Raise an exception when model without primary key calls .find_with_ids
Merge pull request #12549 from makimoto/raise-when-find-without-pk

Raise an exception when model without primary key calls .find_with_ids