Question

Trying to understand Ruby a bit better, I ran into this code surfing the Internet:

require 'rubygems'
require 'activeresource'



ActiveResource::Base.logger = Logger.new("#{File.dirname(__FILE__)}/events.log")

class Event < ActiveResource::Base
  self.site = "http://localhost:3000"
end

events = Event.find(:all)
puts events.map(&:name)

e = Event.find(1)
e.price = 20.00
e.save

e = Event.create(:name      => "Shortest event evar!", 
                 :starts_at => 1.second.ago,
                 :capacity  => 25,
                 :price     => 10.00)
e.destroy

What I’m particularly interested in is how does events.map(&:name) work? I see that events is an array, and thus it’s invoking its map method. Now my question is, where is the block that’s being passed to map created? What is the symbol :name in this context? I’m trying to understand how it works.

Answer 1

events.map(&:name)

is exactly equivalent to

events.map{|x| x.name}

it is just convenient syntactic sugar.

For more details, check out the Symbol#to_proc method here. Here, :name is being coerced to a proc.

By the way, this comes up often here – it’s just very hard to google or otherwise search for ‘the colon thing with an ampersand’ :).