In Ruby 2.1.0 or earlier, the reject method in any class that inherits Hash returns an object of its own class. However, in Ruby 2.1.1 this behavior has changed accidentally to return a plain Hash object, not of the inherited class.

class SubHash < Hash; end
p SubHash.new.reject{}.class #=> 2.1.0: SubHash 2.1.1: Hash
p Hash.new.reject{}.class #=> 2.1.0: Hash 2.1.1: Hash

(To be exact, extra states such as instance variables, etc aren't copied either. With the release of Ruby 2.1.0 we've changed our version policy, so 2.1.1 shouldn't include such behavior change. )

This regression could potentially affect many libraries, one such case is Rails' HashWithIndifferentAccess and OrderedHash. They are broken; as the reject method now returns a plain Hash instead of HashWithIndifferentAccess or OrderedHash. https://github.com/rails/rails/issues/14188

Why is this happened

Firstly, this is not an expected change. It's an accident due to one missing backport commit into 2.1.1.

This behavior change was originally discussed in bugs.r-l.o#9223. However, it had been rejected for the release of 2.1.0 because it was too late. So, this change was rescheduled for Ruby 2.2.0, with a deprecation warning added to 2.1.0.

Commits around this change are described here, read the following gist for more detail: https://gist.github.com/sorah/9265008

Ruby 2.1.0 contains a constant in C to switch Hash#reject behavior by using #ifdef. Hash#reject will return a plain Hash by setting this C constant to 0. When this constant is set to 1, Hash#reject will return the object of its class with any extra states.

After checking out the 2.1 branch, the revision 44358 changed this constant name, and was backported to the 2.1 branch. However, this commit had leaked one line which changed the constant name. This leak is fixed in revision 44370, but this was not included in the backport to the 2.1 branch. Yes, this is reason of the regression.

Accident.

So I recommend to build a patched Ruby 2.1.1 with revision 44370 or add this monkey patch to your application: https://github.com/rails/rails/pull/14198/files

As of now, revision 44370 is backported to the 2.1 branch, so this accidental behavior change will be fixed when Ruby 2.1.2 is released. https://bugs.ruby-lang.org/issues/9576

As I wrote above, this behavior change is still scheduled for the release of Ruby 2.2.0. I recommend to fix your code to in order to expect this behavior change. One option is to re-define the reject method to your class like Rails pull#14198 does.

(This article is translation of my Japanese blog with request and help from zzak <3. Thank you for proofreading, zzak!: http://diary.sorah.jp/2014/02/28/ruby211-hash-reject)