Building Hash by grouping array of objects based on a property of the items

I'm wondering if there is a more canonical way to do this in ruby 1.9

I have an array with a bunch of objects and I want to group them into a Hash using a property of each object in the array.

Very simplified example:

> sh = {}
 => {} 
> aers = %w(a b c d ab bc de abc)
 => ["a", "b", "c", "d", "ab", "bc", "de", "abc"] 
> aers.each do |aer|
>     sh[aer.size] = [] if sh[aer.size].nil?
>     sh[aer.size] << aer
>   end
=> ["a", "b", "c", "d", "ab", "bc", "de", "abc"] 
> sh
 => {1=>["a", "b", "c", "d"], 2=>["ab", "bc", "de"], 3=>["abc"]} 

I tried this, but its output is wrong (as you can see):

 sh = Hash.new([])
 => {} 
> aers.each do |aer|
>     sh[aer.size] << aer
>   end
 => ["a", "b", "c", "d", "ab", "bc", "de", "abc"] 
> sh
 => {}