gistfile1.rb

#======== Ruby 1.9.3 ========================

# Order of items with duplicated values will change if array contains items with other values
1.9.3-p547 :036 >   arr = [{:id=>1, :val=>30}, {:id=>2, :val=>30}, {:id=>1002, :val=>82}]
 => [{:id=>1, :val=>30}, {:id=>2, :val=>30}, {:id=>1002, :val=>82}] 
1.9.3-p547 :037 > arr.sort{|x,y| x[:val] <=> y[:val]}.map{|x| x[:id]}
 => [1, 2, 1002] 
1.9.3-p547 :038 > arr.sort{|x,y| y[:val] <=> x[:val]}.map{|x| x[:id]}
 => [1002, 2, 1] 

# Order of items with duplicated values won't change if array doesn't contain items with other values
1.9.3-p547 :039 > arr = [{:id=>1, :val=>30}, {:id=>2, :val=>30}]
 => [{:id=>1, :val=>30}, {:id=>2, :val=>30}] 
1.9.3-p547 :040 > arr.sort{|x,y| x[:val] <=> y[:val]}.map{|x| x[:id]}
 => [1, 2] 
1.9.3-p547 :041 > arr.sort{|x,y| y[:val] <=> x[:val]}.map{|x| x[:id]}
 => [1, 2] 

#======== Ruby 2.2.0 ========================

# Order of items with duplicated values won't change if array contains items with other values
2.2.0 :021 >   arr = [{:id=>1, :val=>30}, {:id=>2, :val=>30}, {:id=>1002, :val=>82}]
 => [{:id=>1, :val=>30}, {:id=>2, :val=>30}, {:id=>1002, :val=>82}] 
2.2.0 :022 > arr.sort{|x,y| x[:val] <=> y[:val]}.map{|x| x[:id]}
 => [1, 2, 1002] 
2.2.0 :023 > arr.sort{|x,y| y[:val] <=> x[:val]}.map{|x| x[:id]}
 => [1002, 1, 2] 

# Order of items with duplicated values won't change if array doesn't contain items with other values
2.2.0 :024 > arr = [{:id=>1, :val=>30}, {:id=>2, :val=>30}]
 => [{:id=>1, :val=>30}, {:id=>2, :val=>30}] 
2.2.0 :025 > arr.sort{|x,y| x[:val] <=> y[:val]}.map{|x| x[:id]}
 => [1, 2] 
2.2.0 :026 > arr.sort{|x,y| y[:val] <=> x[:val]}.map{|x| x[:id]}
 => [1, 2]