Don't calculate all changes just to check if the param field has changed

[mhodgson]

Starting with Rails 4.2, calculating the changes hash in activerecord is much more expensive since it now tracks in-place changes in json/jsonb/hstore fields. We noticed a ~10x hit to performance due to these changes being calculated on every to_param call on some of our models with large json fields.

This change simply checks for changes in a less expensive way before calculating the changes hash if the param field has indeed changed.

All tests still pass and I didn't see a need to add additional testing.

[coveralls]

Coverage decreased (-0.31%) to 98.44% when pulling e1f83f7 on GoBoundless:prevent-changes into ccedf1a on norman:master.

[coveralls]

Coverage decreased (-0.31%) to 98.44% when pulling e1f83f7 on GoBoundless:prevent-changes into ccedf1a on norman:master.

[norman]

Merged, thanks!

[mhodgson]

Of course, thanks for getting it merged quickly!

[norman]

This is now out in 5.2.0.beta.1. No timetable for stable release yet, but probably within a couple weeks.