How to replace a hash key with another key

hash[:new_key] = hash.delete :old_key

rails Hash has standard method for it:

hash.transform_keys{ |key| key.to_s.upcase }

http://api.rubyonrails.org/classes/Hash.html#method-i-transform_keys

UPD: ruby 2.5 method

If all the keys are strings and all of them have the underscore prefix, then you can patch up the hash in place with this:

h.keys.each { |k| h[k[1, k.length - 1]] = h[k]; h.delete(k) }

The k[1, k.length - 1] bit grabs all of k except the first character. If you want a copy, then:

new_h = Hash[h.map { |k, v| [k[1, k.length - 1], v] }]

Or

new_h = h.inject({ }) { |x, (k,v)| x[k[1, k.length - 1]] = v; x }

You could also use sub if you don't like the k[] notation for extracting a substring:

h.keys.each { |k| h[k.sub(/\A_/, '')] = h[k]; h.delete(k) }Hash[h.map { |k, v| [k.sub(/\A_/, ''), v] }]h.inject({ }) { |x, (k,v)| x[k.sub(/\A_/, '')] = v; x }

And, if only some of the keys have the underscore prefix:

h.keys.each do |k|  if(k[0,1] == '_')    h[k[1, k.length - 1]] = h[k]    h.delete(k)  endend

Similar modifications can be done to all the other variants above but these two:

Hash[h.map { |k, v| [k.sub(/\A_/, ''), v] }]h.inject({ }) { |x, (k,v)| x[k.sub(/\A_/, '')] = v; x }

should be okay with keys that don't have underscore prefixes without extra modifications.