mongoid/mongoid — pull request #3773 by ElMassimo (2014-08-14 18:56:08+00:00)
Title: Use nanoseconds for cache_key timestamp instead of plain seconds

This has caused several issues for our app, specially in the test environment.
I think it would be better to define the cache key using nanoseconds, as in:
```ruby
def cache_key
...
return "#{model_key}/#{id}-#{updated_at.utc.to_s(:nsec)}" if do_or_do_not(:updated_at)
...
end
```

Fixes #3772.