An eh solution for SO Question#7400657.rb

purchase_order_test.rb

class PurchaseOrderTest < ActiveSupport::TestCase
  context "when PurchaseOrder exists" do
    should "use the existing PurchaseOrder" do
      original = PurchaseOrder.create! :purchase_order_no => "abcde", :note => "Original"
      new_po   = PurchaseOrder.create! :purchase_order_no => "abcde", :note => "New"

      assert_equal 1, PurchaseOrder.all.size
      assert_equal "Original", new_po.note
    end
  end

  context "SalesTransaction" do
    should "allow SalesTransactions to be created through anaf" do
      original = PurchaseOrder.create! :purchase_order_no => "abcde", :note => "Original"
      new_po   = PurchaseOrder.create :purchase_order_no => "abcde", 
        :sales_transactions_attributes => [
          {:note => "Sale 1"},
          {:note => "Sale 2"}
        ]

      assert_equal 1, PurchaseOrder.all.size
      assert_equal original.note, new_po.note
      assert_equal 2, new_po.sales_transactions.size
    end
  end
end

PurchaseOrder.rb

class PurchaseOrder < ActiveRecord::Base
  has_many :sales_transactions, :foreign_key => "po_purchase_order_no", 
    :primary_key => "purchase_order_no"

  before_validation :check_for_exisitng_po # maybe only on create?

  accepts_nested_attributes_for :sales_transactions

  private

  def check_for_exisitng_po
    existing_po = PurchaseOrder.find_by_purchase_order_no(self.purchase_order_no)

    if existing_po
      self.id = existing_po.id
      self.reload # don't like this, also will overwrite new attrs
      @new_record = false # hmm...
    end

    true
  end
end

sales_transaction_test.rb

class SalesTransactionTest < ActiveSupport::TestCase
  context "PurchaseOrder" do
    should "allow PurchaseOrders to be created through anaf" do
      original = PurchaseOrder.create! :purchase_order_no => "abcde", :note => "Original"
      sale = SalesTransaction.create! :note => "Sale 1", 
        :purchase_order_attributes => {:purchase_order_no => "abcde", :note => "New"}

      assert_equal 1, PurchaseOrder.all.size
      assert_equal original.note, sale.purchase_order.note
    end
  end
end

SalesTransaction.rb

class SalesTransaction < ActiveRecord::Base
  belongs_to :purchase_order, :foreign_key => "po_purchase_order_no", 
    :primary_key => "purchase_order_no"

  accepts_nested_attributes_for :purchase_order
end