Allow transfers based on transactions in different currencies (#903)

* Allow transfers between transactions in different currencies

* Review fixes

app/models/account/transfer.rb

@@ -1,7 +1,8 @@
 class Account::Transfer < ApplicationRecord
   has_many :transactions, dependent: :nullify
 
-  validate :transaction_count, :from_different_accounts, :net_zero_flows, :all_transactions_marked
+  validate :net_zero_flows, if: :single_currency_transfer?
+  validate :transaction_count, :from_different_accounts, :all_transactions_marked
 
   def inflow_transaction
     transactions.find { |t| t.inflow? }
@@ -32,6 +33,10 @@ class Account::Transfer < ApplicationRecord
 
   private
 
+    def single_currency_transfer?
+      transactions.map(&:currency).uniq.size == 1
+    end
+
     def transaction_count
       unless transactions.size == 2
         errors.add :transactions, "must have exactly 2 transactions"

test/models/account/transfer_test.rb

@@ -39,11 +39,18 @@ class Account::TransferTest < ActiveSupport::TestCase
     end
   end
 
-  test "transfer transactions must net to zero" do
+  test "single-currency transfer transactions must net to zero" do
     @outflow.update! amount: 105
 
     assert_raises ActiveRecord::RecordInvalid do
       Account::Transfer.create! transactions: [ @inflow, @outflow ]
     end
   end
+
+  test "multi-currency transfer transactions do not have to net to zero" do
+    @outflow.update! amount: 105, currency: "EUR"
+    transfer = Account::Transfer.create! transactions: [ @inflow, @outflow ]
+
+    assert transfer.valid?
+  end
 end