# In lib/puppet/node.rb:
class Puppet::Node
    ...
    # Set up indirection, so that nodes can be looked for in
    # the node sources.
    require 'puppet/indirector' 
    extend Puppet::Indirector

    # Use the node source as the indirection terminus.
    indirects :node, :to => :node_source
    ...
end

# Then something else uses this indirection:
if node = Puppet::Node.get("mynode")
  puts node.environment
end