Ruby におけるハッシュ (Hash) と構造体 (Struct) の使い分け ref: http://qiita.com/megane42/items/5bfdb0764fa575efbdab

file0.rb

# ハッシュ
person = {name: "Taro", age: 16}
p person[:name]

# 構造体
Person = Struct.new(:name, :age)
person = Person.new("Taro", 16)
p person.name

file1.rb

# ハッシュ
person = {name: "Taro", age: 16}
person[:tel] # => nil
person[:sex] = "man" # 要素が追加される

# 構造体
Person = Struct.new(:name, :age)
person = Person.new("Taro", 16)
person.tel # => NameError: undefined method 'tel'
person.sex = "man" # => NameError: undefined method 'sex='

file2.rb

key = ARGV[0]

# ハッシュ
p some_hash[key] # => なんとかなる

# 構造体
p some_struct.key # => ムリ
p some_struct.send(key) # => なんとかなるけど気持ち悪い

file3.rb

Person = Struct.new(:name, :age)
person = Person.new("Taro", 16)

Robot = Struct.new(:name, :age)
robot = Robot.new("Taro", 16)

p person == robot # => false. 越えられないカベ

file4.rb

# ハッシュ
person.to_a
=> [[:name, "Taro"], [:age, 16]]

# 構造体
person.to_a
=> ["taro", 16]

file5.rb

# ハッシュ
person1 = {name: "Taro", age: 16, most_favorite_movie_ever_seen: "Toy Story"}
person2 = {name: "Jiro", age: 17, most_favorite_movie_ever_seen: "Toy Story 2"}
person3 = {name: "Maro", age: 18, most_favorite_movie_ever_seen: "Toy Story 3"}

# 構造体
Person = Struct.new(:name, :age, :most_favorite_movie_ever_seen)
person1 = Person.new("Taro", 16, "Toy Story")
person2 = Person.new("Jiro", 17, "Toy Story 2")
person3 = Person.new("Maro", 18, "Toy Story 3")

hoge.rb

require "open-uri"

class Hoge
  def call_api
    response = open "http://something-api/"
    response.read if response.status[0] == "200"
  end
end

hoge_spec.rb

it '接続成功時は "Success" を返す' do
  hoge = Hoge.new

  # read と status に応答できるオブジェクトを即席で作る
  dummy = Struct.new(:read, :status).new('{"result": "Success"}', ["200", "OK"])
  # dummy を返すように open メソッドをスタブ化
  allow(hoge).to receive(:open).and_return(dummy)

  expect(hoge.call_api).to eq '{"result": "Success"}'
end