Ruby: merge nested hash

I would like to merge a nested hash.

a = {:book=>
    [{:title=>"Hamlet",
      :author=>"William Shakespeare"
      }]}

b = {:book=>
    [{:title=>"Pride and Prejudice",
      :author=>"Jane Austen"
      }]}

I would like the merge to be:

{:book=>
   [{:title=>"Hamlet",
      :author=>"William Shakespeare"},
    {:title=>"Pride and Prejudice",
      :author=>"Jane Austen"}]}

What is the nest way to accomplish this?

For rails 3.0.0+ or higher version there is the deep_merge function for ActiveSupport that does exactly what you ask for.

I found a more generic deep-merge algorithm here, and used it like so:

class ::Hash
    def deep_merge(second)
        merger = proc { |key, v1, v2| Hash === v1 && Hash === v2 ? v1.merge(v2, &merger) : v2 }
        self.merge(second, &merger)
    end
end

a.deep_merge(b)

To add on to Jon M and koendc's answers, the below code will handle merges of hashes, and :nil as above, but it will also union any arrays that are present in both hashes (with the same key):

class ::Hash
  def deep_merge(second)
    merger = proc { |_, v1, v2| Hash === v1 && Hash === v2 ? v1.merge(v2, &merger) : Array === v1 && Array === v2 ? v1 | v2 : [:undefined, nil, :nil].include?(v2) ? v1 : v2 }
    merge(second.to_h, &merger)
  end
end


a.deep_merge(b)

For variety's sake - and this will only work if you want to merge all the keys in your hash in the same way - you could do this:

a.merge(b) { |k, x, y| x + y }

When you pass a block to Hash#merge, k is the key being merged, where the key exists in both a and b, x is the value of a[k] and y is the value of b[k]. The result of the block becomes the value in the merged hash for key k.

I think in your specific case though, nkm's answer is better.