A deep_reject method for hashes in Ruby

Recently, while adding missing functionality for the i18n-js gem I’ve stumbled into a problem. I needed to have a method to “deep reject” keys in the hash. There are some examples in the wild doing that, but they all solve this problem by adding a new method to the Hash class. I wanted a generic method, which would take the hash as an argument.

After some head scratching, tinkering and tweaking, I’ve come up with a correct solution (at least I think it’s correct). Here it is:

def self.deep_reject(hash, &block)                                                   
  hash.each_with_object({}) do |(k, v), memo|                                        
    unless block.call(k, v)                                                          
      if v.is_a?(Hash)                                                               
        memo[k] = deep_reject(v, &block)                                             
      else                                                                           
        memo[k] = v                                                                  
      end                                                                            
    end                                                                              
  end                                                                                
end                                                                                  

You use it like this:

hash = {:a => {:b => 1, :c => 2}}

result = deep_reject(hash) { |k, v| k == :b }

result # => {:a => {:c => 2}}