The cookies object in your controllers and views is a ActionController::CookieJar and even though that class inherits from Hash and often behaves like one, you can not call slice on it to select only a subset of cookies. Why? Because Hash#slice calls self.class.new -- which in case of a CookieJar just won't work.

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Unfortunately, you can't even say cookies.to_hash and slice on that, just because CookieJar#to_hash is inherited from Hash and will just return self. Bummer.

You need to do it yourself, for example by using collect_hash:

my_keys = %w[ foo bar baz ]
my_cookies = my_keys.collect_hash { |key| [ key, cookies[key] ] }