Recursively Symbolize Keys

Given a YAML stream that looks like this:

---
drinks:
  martini:
    garnish: olive
  gibson:
    garnish: onion

The Ruby-fied version looks like this:

{"drinks"=>{"gibson"=>{"garnish"=>"onion"}, "martini"=>{"garnish"=>"olive"}}}

But we don’t like string keys, we like symbol keys. A number of libraries exist to extend Hash with interchangeable string- or key-based indexing. But adding a library dependency seems like overkill for symbolizing some keys. Here’s a quick recursive string-to-symbol key converter:

def symbolize_keys(hash)
  hash.inject({}){|result, (key, value)|
    new_key = case key
              when String then key.to_sym
              else key
              end
    new_value = case value
                when Hash then symbolize_keys(value)
                else value
                end
    result[new_key] = new_value
    result
  }
end

Note the block parameters to Hash#inject.

  hash.inject({}){|result, (key, value)|

The block arguments would normally be a hash (the accumulator) and a two-element array of [key, value]. Putting parentheses around the second two parameters causes the second argument to be “splatted” into its component parts and assigned to key and value separately.

Let’s take a look at the output.

puts "Symbolized hash:"
puts symbolize_keys(YAML.load(yaml)).inspect

Symbolized hash:
{:drinks=>{:martini=>{:garnish=>"olive"}, :gibson=>{:garnish=>"onion"}}}

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