Question

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What does map(&:name) mean in Ruby?

I was watching a railscast and saw this code.

[Category, Product].(&:delete_all) 

In regards to clearing a database.

I asked about the line in IRC and was told

(&:delete_all) 

was a shortcut for

{|model| model.delete_all} 

I tested this with the following

class ClassOne   def class_method     puts 1   end end  class ClassTwo   def class_method     puts 2   end end  [ClassOne, ClassTwo].each(&:class_method) 

I received an error saying

Wrong Argument type Symbol (expected Proc) 

I also tried

one = ClassOne.new two = ClassTwo.new  [one, two].each(&:class_method) 

But that still failed.

If I modified it to read

[one, two].each{|model| model.class_method} 

Everything worked as expected.

So, what does &:delete_all actually do? The docs say delete_all is a method, so I am confused as to what is going on here.

Answer 1

This relies upon a Ruby 1.9 extension that can be done in 1.8 by including the following:

class Symbol     def to_proc       proc { |obj, *args| obj.send(self, *args) }     end end 

I believe Rails defines this in ActiveSupport.