#Instead of using:
books  = Book.find(:all, :order => 'id desc', :limit => 20)
comics = []
books.each do |book|
  comics << book if book.item_type == 'Comic'
end

#OR
comics  = Book.find(:all, :order => 'id desc', :limit => 20).map {|book| [book] unless book.item_type == 'Comic'}.flatten!

#Use this:
books  = Book.find(:all, :order => 'id desc', :limit => 20)
comics = books.select { |i| i.item_type == 'Comic' }

http://errtheblog.com/post/608

Useful for when you need to do a find and map specific results to an array for use later

Using Enumerable#select to create a specific array

rails, ruby

Ruby