How to merge two hashes that have same keys in ruby

If you're using Active Support (Rails), which adds Hash#transform_values, I really like this easy-to-read solution when you have n hashes:

hashes = [hash_1, hash_2, hash_3] # any number of hashes
hashes.flat_map(&:to_a).group_by(&:first).transform_values { |x| x.sum(&:last) }

Use Hash#merge or Hash#merge!:

a = {a: 1, b: 2, c: 3}
b = {a: 2, c: 4, b: 3}
a.merge!(b) { |k, o, n| o + n }
a # => {:a=>3, :b=>5, :c=>7}

The block is called with key, old value, new value. And the return value of the block is used as a new value.

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