As explained here (https://groups.google.com/forum/#!topic/mongoid/pREjqqSSnMU), mongoid seems to ignore nested attributes validation errors for existing (updated) nested documents.

I had an answer on StackOverflow (http://stackoverflow.com/questions/11542244/rails-mongoid-update-attributes-ignoring-persisted-nested-attributes-validations/11688456#11688456) sending me to a fork commit that seems to fix this issue (didn't try yet...the fork is based on a very old version of mongoid 3 => I'll write a monkey patch since the fix is very simple).

Here is the commit : https://github.com/jiren/mongoid/commit/9ebb8af8514ea70c66a7dbe64be20bc7407a2829 (only custom code in the fork)

Maybe you can check if the fix is good enough (perf, ...) to be included in official repo ?

Thanks !