Ruby on Rails: a simpler Hash.slice method?

Question

In http://api.rubyonrails.org/classes/Hash.html#method-i-slice, I see

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

I don't get the usage here. I mean, if I pass an array into slice, I definitely want every key get sliced, then why use an addtional * here?

Is is possible to make it into something like:

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

by creating a simpler version of slice?

Answer 1

Hashes can have all sorts of things for keys, the keys don't have to be just strings or symbols. For example, this is a perfectly valid Hash:

{ [ :mass, :velocity, :time ] => 'pancakes' }

How is Hash#slice supposed to know that sometimes this:

h.slice([:mass, :velocity, :time])

means "give me the sub-hash with those three keys" and sometimes it means "give me the sub-hash with that array as a key"?

Hash#slice takes a list of keys (one key per argument) so if you want to have your keys in an array then you have to splat them into individual arguments:

h.slice(*some_array_of_keys)
# ------^ expand the array into individual arguments

That's why you need the *.

If you want to assume that the keys will never be arrays then you're free to say something like:

def simple_slice(*keys) # Allow any number of arguments
  slice(*keys.flatten)  # flatten all the arrays and then punt to Rails's slice.
end

You might not want to flatten only one level (or maybe you don't).