Refactor Plutus::Transaction validation to use AmountsExtension

This reduces the number of different pieces of code that sums
Amounts, incidentally also catching a previously unnoticed bug in
Plutus::Transaction#difference_of_amounts, captured in the updated
spec.

app/models/plutus/amounts_extension.rb

@@ -5,7 +5,11 @@ module AmountsExtension
     def balance
       balance = BigDecimal.new('0')
       each do |amount_record|
-        balance += amount_record.amount
+        if amount_record.amount
+          balance += amount_record.amount
+        else
+          balance = nil
+        end
       end
       return balance
     end

app/models/plutus/transaction.rb

@@ -25,8 +25,8 @@ class Transaction < ActiveRecord::Base
     attr_accessible :description, :commercial_document
 
     belongs_to :commercial_document, :polymorphic => true
-    has_many :credit_amounts
-    has_many :debit_amounts
+    has_many :credit_amounts, :extend => AmountsExtension
+    has_many :debit_amounts, :extend => AmountsExtension
     has_many :credit_accounts, :through => :credit_amounts, :source => :account
     has_many :debit_accounts, :through => :debit_amounts, :source => :account
 
@@ -71,13 +71,7 @@ def has_debit_amounts?
       end
 
       def amounts_cancel?
-        errors[:base] << "The credit and debit amounts are not equal" if difference_of_amounts != 0
-      end
-
-      def difference_of_amounts
-        credit_amount_total = credit_amounts.inject(0) {|sum, credit_amount| sum + credit_amount.amount.to_i}
-        debit_amount_total = debit_amounts.inject(0) {|sum, debit_amount| sum + debit_amount.amount.to_i}
-        credit_amount_total - debit_amount_total  
+        errors[:base] << "The credit and debit amounts are not equal" if credit_amounts.balance != debit_amounts.balance
       end
   end
 end

spec/models/transaction_spec.rb

@@ -52,6 +52,14 @@ module Plutus
       transaction.errors['base'].should == ["The credit and debit amounts are not equal"]
     end
 
+    it "should require the debit and credit amounts to cancel even with fractions" do
+      transaction = FactoryGirl.build(:transaction)
+      transaction.credit_amounts << FactoryGirl.build(:credit_amount, :amount => 100.1, :transaction => transaction)
+      transaction.debit_amounts << FactoryGirl.build(:debit_amount, :amount => 100.2, :transaction => transaction)
+      transaction.should_not be_valid
+      transaction.errors['base'].should == ["The credit and debit amounts are not equal"]
+    end
+
     it "should have a polymorphic commercial document associations" do
       mock_document = FactoryGirl.create(:asset) # one would never do this, but it allows us to not require a migration for the test
       transaction = FactoryGirl.build(:transaction_with_credit_and_debit, :commercial_document => mock_document)