Add convert currency and tests

app/models/currency_conversion.rb

@@ -25,4 +25,15 @@ class CurrencyConversion < ApplicationRecord
   belongs_to :conference
   validates :rate, numericality: { greater_than: 0 }
   validates :from_currency, uniqueness: { scope: :to_currency }, on: :create
+
+  def self.convert_currency(conference, amount, from_currency, to_currency)
+    conversion = conference.currency_conversions.find_by(from_currency: from_currency, to_currency: to_currency)
+    if conversion
+      Money.add_rate(from_currency, to_currency, conversion.rate)
+      amount.exchange_to(to_currency)
+    else
+      # If no conversion is found. Should not be possible with the currency design of fixed allowed currencies.
+      amount
+    end
+  end
 end

spec/models/currency_conversion_spec.rb

@@ -24,6 +24,7 @@
 
 describe CurrencyConversion do
   let!(:conference) { create(:conference, title: 'ExampleCon') }
+  let(:amount) { Money.from_amount(10.00, 'USD') }
 
   describe 'validations' do
     before do
@@ -34,4 +35,26 @@
 
     it { is_expected.to validate_uniqueness_of(:from_currency).scoped_to(:to_currency).on(:create) }
   end
+
+  describe 'convert currency functionality' do
+    context 'when conversion rate exists' do
+      before do
+        conference.currency_conversions << create(:currency_conversion, from_currency: 'USD', to_currency: 'EUR', rate: 0.89)
+        conference.currency_conversions << create(:currency_conversion, from_currency: 'USD', to_currency: 'GBP', rate: 0.75)
+      end
+
+      it 'converts currency using existing rate' do
+        converted_amount = described_class.convert_currency(conference, amount, 'USD', 'EUR')
+        expect(converted_amount.currency).to eq('EUR')
+        expect(converted_amount.cents).to eq(890)
+      end
+    end
+
+    context 'when conversion rate does not exist' do
+      it 'returns the original amount if no conversion is found' do
+        original_amount = described_class.convert_currency(conference, amount, 'USD', 'INR')
+        expect(original_amount.cents).to eq(1000)
+      end
+    end
+  end
 end