mongoid/mongoid — pull request #1904 by Empact (2012-04-11 10:54:46+00:00)
Title: Failing spec shows that parent#collection#concat on a new parent followed by parent#save persists the concated docs when collection is embedded but doesn't when it's referenced

Personally I find this "silent failure" behavior unintuitive. I'd be happy to write a fix if you guys agree. I'd prefer either failing noisily by raising, or else saving at that point.