Handle missing exchange rate provider, allow fallback for missing rates (#955)

* Clean up exchange rate logic

* Remove stale method

lib/money.rb

@@ -2,7 +2,10 @@ class Money
   include Comparable, Arithmetic
   include ActiveModel::Validations
 
-  attr_reader :amount, :currency
+  class ConversionError < StandardError
+  end
+
+  attr_reader :amount, :currency, :store
 
   validate :source_must_be_of_known_type
 
@@ -16,20 +19,27 @@ class Money
     end
   end
 
-  def initialize(obj, currency = Money.default_currency)
+  def initialize(obj, currency = Money.default_currency, store: ExchangeRate)
     @source = obj
     @amount = obj.is_a?(Money) ? obj.amount : BigDecimal(obj.to_s)
     @currency = obj.is_a?(Money) ? obj.currency : Money::Currency.new(currency)
+    @store = store
 
     validate!
   end
 
-  # TODO: Replace with injected rate store
-  def exchange_to(other_currency, date = Date.current)
-    if currency == Money::Currency.new(other_currency)
+  def exchange_to(other_currency, date: Date.current, fallback_rate: nil)
+    iso_code = currency.iso_code
+    other_iso_code = Money::Currency.new(other_currency).iso_code
+
+    if iso_code == other_iso_code
       self
-    elsif rate = ExchangeRate.find_rate(from: currency, to: other_currency, date: date)
-      Money.new(amount * rate.rate, other_currency)
+    else
+      exchange_rate = store.find_rate(from: iso_code, to: other_iso_code, date: date)&.rate || fallback_rate
+
+      raise ConversionError.new("Couldn't find exchange rate from #{iso_code} to #{other_iso_code} on #{date}") unless exchange_rate
+
+      Money.new(amount * exchange_rate, other_iso_code)
     end
   end