Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

test/models/exchange_rate_test.rb

@@ -1,7 +1,42 @@
 require "test_helper"
 
 class ExchangeRateTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  test "find rate in db" do
+    assert_equal exchange_rates(:day_29_ago_eur_to_usd),
+      ExchangeRate.find_rate_or_fetch(from: "EUR", to: "USD", date: 29.days.ago.to_date)
+  end
+
+  test "fetch rate from provider when it's not found in db" do
+    ExchangeRate
+      .expects(:fetch_rate_from_provider)
+      .returns(ExchangeRate.new(base_currency: "USD", converted_currency: "MXN", rate: 1.0, date: Date.current))
+
+    ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+  end
+
+  test "provided rates are saved to the db" do
+    VCR.use_cassette "synth_exchange_rate" do
+      assert_difference "ExchangeRate.count", 1 do
+        ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+      end
+    end
+  end
+
+  test "retrying, then raising on provider error" do
+    Faraday.expects(:get).returns(OpenStruct.new(success?: false)).times(3)
+
+    error = assert_raises Provider::Base::ProviderError do
+      ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+    end
+
+    assert_match "Failed to fetch exchange rate from Provider::Synth", error.message
+  end
+
+  test "retrying, then raising on network error" do
+    Faraday.expects(:get).raises(Faraday::TimeoutError).times(3)
+
+    assert_raises Faraday::TimeoutError do
+      ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+    end
+  end
 end