Hash remove all except specific keys

I would like to remove every key from a hash except a given key.

For example:

{
 "firstName": "John",
 "lastName": "Smith",
 "age": 25,
 "address":
 {
     "streetAddress": "21 2nd Street",
     "city": "New York",
     "state": "NY",
     "postalCode": "10021"
 },
 "phoneNumber":
 [
     {
       "type": "home",
       "number": "212 555-1234"
     },
     {
       "type": "fax",
       "number": "646 555-4567"
     }
 ]
}

I want to remove everything except "firstName" and/or "address"

Thanks

What about slice?

hash.slice('firstName', 'lastName')
 # => { 'firstName' => 'John', 'lastName' => 'Smith' }

Some other options:

h.select {|k,v| ["age", "address"].include?(k) }

Or you could do this:

class Hash
  def select_keys(*args)
    select {|k,v| args.include?(k) }
  end
end

So you can now just say:

h.select_keys("age", "address")