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

app/models/exchange_rate.rb

@@ -1,42 +1,22 @@
 class ExchangeRate < ApplicationRecord
+  include Provided
+
   validates :base_currency, :converted_currency, presence: true
 
   class << self
-    def convert(from, to, amount)
-      rate = ExchangeRate.find_by(base_currency: from, converted_currency: to, date: Date.current)
-      return nil if rate.nil?
-      amount * rate.rate
+    def find_rate(from:, to:, date:)
+      find_by \
+        base_currency: Money::Currency.new(from).iso_code,
+        converted_currency: Money::Currency.new(to).iso_code,
+        date: date
     end
 
-    def get_rate(from, to, date)
-      _from = Money::Currency.new(from)
-      _to = Money::Currency.new(to)
-      find_by! base_currency: _from.iso_code, converted_currency: _to.iso_code, date: date
-    rescue
-      logger.warn "Exchange rate not found for #{_from.iso_code} to #{_to.iso_code} on #{date}"
-      nil
+    def find_rate_or_fetch(from:, to:, date:)
+      find_rate(from:, to:, date:) || fetch_rate_from_provider(from:, to:, date:).tap(&:save!)
     end
 
     def get_rate_series(from, to, date_range)
       where(base_currency: from, converted_currency: to, date: date_range).order(:date)
     end
-
-    # TODO: Replace with generic provider
-    # See https://github.com/maybe-finance/maybe/pull/556
-    def fetch_rate_from_provider(from, to, date)
-      response = Faraday.get("https://api.synthfinance.com/rates/historical") do |req|
-        req.headers["Authorization"] = "Bearer #{ENV["SYNTH_API_KEY"]}"
-        req.params["date"] = date.to_s
-        req.params["from"] = from
-        req.params["to"] = to
-      end
-
-      if response.success?
-        rates = JSON.parse(response.body)
-        rates.dig("data", "rates", to)
-      else
-        nil
-      end
-    end
   end
 end