.collect(&:id)的语法问题

[liangchaob]

在研究 Rails 102 的时候，尝试.collect(&:id)的语法结构貌似不认这个语法结构：

2.3.1 :047 > topics = [{id:1, title:"topic1"}, {id:2, title:"topic2"}]
 => [{:id=>1, :title=>"topic1"}, {:id=>2, :title=>"topic2"}]
2.3.1 :048 > topics.collect(&:id)
NoMethodError: undefined method `id' for {:id=>1, :title=>"topic1"}:Hash
    from (irb):48:in `collect'
    from (irb):48
    from /Users/alan/.rvm/rubies/ruby-2.3.1/bin/irb:11:in `<main>'

试过 stackoverflow 中的不含hash的花括号结构是有效的：

2.3.1 :038 >   a = [ "a", "b", "c", "d" ]
 => ["a", "b", "c", "d"]
2.3.1 :039 > a.collect {|x| x + "!" }
 => ["a!", "b!", "c!", "d!"]

是不是ruby语法结构变化又不兼容了呢，array里面套hash后collect方式是怎样的呢？