#006_用to_proc标记缩写代码块

我们经常需要将一个代码块传递给迭代器，并在代码块中调用另一个方法。Rails提供了一条实现这一功能的捷径，于是下面两句代码等价：

groups = posts.group_by { |post| post.author_id }
groups = posts.group_by(&:author_id)

让我们看下这一切是如何工作的:

class Symbol   
  # Turns the symbol into a simple proc, which is especially useful for enumerations. Examples:   
  #   
  #   # The same as people.collect { |p| p.name }   
  #   people.collect(&:name)   
  #   
  #   # The same as people.select { |p| p.manager? }.collect { |p| p.salary }   
  #   people.select(&:manager?).collect(&:salary)   
  def to_proc   
    Proc.new { |*args| args.shift.__send__(self, *args) }   
  end   
end  

&符号用在symbol前面实际上是调用了to_proc方法，而to_proc里返回一个Proc对象，内部为调用symbol指定的方法。举例如下：

projects = Project.find(:all)
projects.collect { |p| p.name }
# => ["Rails Projects", "Housework"]
projects.collect(&:name)
# => ["Rails Projects", "Housework"]
projects.collect(&:name).collect(&:downcase)
# => ["rails projects", "housework"]
projects.all?(&:valid?)
# => true
projects.any?(&:valid?)
# => true