Chef library which depends on pg gem

cluster.rb

# <cookbook>/libraries/cluster.rb
begin
  require 'pg'
rescue LoadError
   # install dependencies using resources
   ...

   # install pg to omnibus
   p = chef_gem 'pg' do
     action :nothing
   end
   p.run_action(:install)

   require 'pg'
end

# my library code which depends on pg