Use reduce to build a hierarchical tree of items (with help of spikegrobstein)

gistfile1.rb

categories = [
  {:id => 3, :name => "MLB", :parent => 2}, 
  {:id => 2, :name => "Baseball", :parent => 1}, 
  {:id => 1, :name => "Sports", :parent => nil}
]

categories.reverse.reduce(nil) do |memo, category|
  memo = {
    :id     => category[:id],
    :name   => category[:name],
    :parent => memo
  }
  memo
end