mergeとmerge!
Hash#mergeは非破壊的ですが、Hash#merge!は破壊的です。!が付いているメソッドは使い方に注意が必要なメソッドです。
a = { "Alice" => 23, "Bobby" => 16, "Chris" => 21, } b = { "Alice" => 15, "David" => 12, } p a #=> {"Alice"=>23, "Chris"=>21, "Bobby"=>16} p b #=> {"Alice"=>15, "David"=>12} p a.merge(b) #=> {"Alice"=>15, "David"=>12, "Bobby"=>16, "Chris"=>21} p a #=> {"Alice"=>23, "Chris"=>21, "Bobby"=>16} p a.merge!(b) #=> {"Alice"=>15, "David"=>12, "Chris"=>21, "Bobby"=>16} p a #=> {"Alice"=>15, "David"=>12, "Chris"=>21, "Bobby"=>16}