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method

slice

v4.2.1 - Show latest stable - Class: Hash
slice(*keys)
public

Slice a hash to include only the given keys. Returns a hash containing the given keys.

{ a: 1, b: 2, c: 3, d: 4 }.slice(:a, :b)
# => {:a=>1, :b=>2}

This is useful for limiting an options hash to valid keys before passing to a method:

def search(criteria = {})
  criteria.assert_valid_keys(:mass, :velocity, :time)
end

search(options.slice(:mass, :velocity, :time))

If you have an array of keys you want to limit to, you should splat them:

valid_keys = [:mass, :velocity, :time]
search(options.slice(*valid_keys))

2Notes

everything is ok

evilguc · Aug 20, 20132 thanks

Olefine, I'm not sure here is a good place for such questions (better use stackoverflow for example), but answer for your question is that Rails provide slice (and many other methods) not only for Hash class but for HashWithIndifferentAccess too such as for any other superclass of Hash, so they use 

hash = self.class.new

for a reason. {} is a literal only for Hash.

unuseful code

Olefine · Aug 12, 2013

Why it use? hash = self.class.new Instead it, we may use hash = {}