The functional core, imperative shell pattern as I see it. I use this quite a bit. Makes the code simpler to understand and test.

gistfile1.rb

# Functional core
# Tested with unit tests. Mostly simple state based tests (like: invoice.gross_amount.should == 200).
class BuildInvoice
  def build
    # build invoice in memory..
  end
end

# Imperative shell
# Usually just tested with a single test to ensure it stores data in the database
class InvoiceCreator
  def self.create(order)
     # load needed data from db
     invoice = BuildInvoice.new(order, ...).build
     # save changes to db
  end
end