`&:views_count` in `Post.published.collect(&:views_count)`

Question

I saw the code from here

Post.published.collect(&:views_count)

I guess it equals to

.collect { |p| p.views_count }

But I never saw this usage before, does this have a name? Where can I find more information about it?

Answer 1

This is actually a rather clever hack made it into ruby 1.9.

Basically, & in front of a variable in ruby coerces it into a proc. It does that by calling to_proc. Some clever fellow (first time I saw this was in _whys code, but I won't credit him cause I don't know if he came up with it) added a to_proc method to Symbol, that is essentially {|obj| obj.send self}.

There aren't many coercians in ruby, but it seems like all of them are mostly used to do hacks like this (like !! to coerce any type into a boolean)

Answer 2

It's a use of Symbol#to_proc. The & operator turns a Proc object into a block, and because Ruby 1.8.7 and newer implement Symbol#to_proc, it can be used with a symbol like :views_count. And yes, it's equivalent to {|p| p.views_count}.