There are many solutions Show archive.org snapshot , but a very concise one is this:

hash.merge!(hash) do |key, old_value, new_value|
  # Return something
end

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The block of merge! will be called whenever a key in the argument hash already exists in the base hash. Since hash is updated with itself, each key will conflict and thus allow you to modify the value of each key to whatever you like (by returning old_value you'd get the behavior of Rails' reverse_merge!, by returning new_value you'd get the behavior of standard merge!).

Note that there are better solutions performance-wise.