chef_as_lib.rb

require 'chef'
require 'chef/data_bag'
require 'chef/data_bag_item'

bag = Chef::DataBag.load("yourbag")
# The "true" here just inflates them to the data bag items
bag.list(true).each do |item|
  item["poop"] = "smith"
  item.save
end

# Or

item = Chef::DataBagItem.load("yourbag", "key")
item["snarf"]
item.save