mongoid/mongoid — comment on commit 5f57f3d39f6a9fbdca157d7d38443b23d42e73da
File: lib/mongoid/finders.rb line 117
By DouweM:

Fair enough. But just like there's no version of `find` that doesn't raise an error, I don't think there should be one for `find_by`. If you want that, you can just use `Model.where(...).first` or `Model.find_by(...) rescue nil`.