Hash Conflict Resolution

hash.rb

hash_a = {foo: 'bar', baz: 'qux'}
hash_b = {foo: 'baz'}

hash_c = hash_a.merge(hash_b) do |key, old, new|
  [old, new]
end

hash_c == {
  foo: ['bar', 'baz'],
  baz: 'qux'
  }