Mapping Object Values With Ruby's Ampersand-symbol Technique

Discovered another little Ruby nugget the other day. The nugget gives a shorter syntax when you want to map the return value of a message sent to a list of objects, say, the name of the class of the object. In the past I would use Array#map to produce the list with something like:

  objects = [1, :number_1, "1"]
  classes = objects.map { |o| o.class }
  classes.inspect
  # => [Fixnum, Symbol, String]

Turns out that Ruby has a shortcut that shortens your keystrokes a bit:

  objects = [1, :number_1, "1"]
  classes = objects.map(&:class)
  classes.inspect
  # => [Fixnum, Symbol, String]

The two snippets are functionally identical. By passing a symbol to map preceded by an ampersand, Ruby will call Symbol#to_proc on the passed symbol (e.g. :class.to_proc ), which returns a proc object like {|o| o.class } . Where would you use this you ask? The day I learned this little ditty I was writing some tests that were verifying some active record associations. Whenever I needed to update values on a has_many collection for a particular model, I actually needed to assert that the associated collection of objects were rebuilt with the new values, deleting the old rows and recreating new ones. The ampersand-symbol technique above was nice for this.

  describe Father do
    it 'should create new children when I attempt to update the children' do
      father = Factory(:father)
      orig_children = father.children.map(&:id)
      # perform the update method
      father.reload
      father.children.map(&:id).should\_not == orig\_children
    end
  end

So I thought I’d pass the word on. Cool stuff in Ruby. Who knew?