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

* Clean up exchange rate logic

* Remove stale method

app/models/exchange_rate.rb

@@ -1,29 +1,29 @@
 class ExchangeRate < ApplicationRecord
   include Provided
 
-  validates :base_currency, :converted_currency, presence: true
+  validates :from_currency, :to_currency, :date, :rate, presence: true
 
   class << self
-    def find_rate(from:, to:, date:)
-      find_by \
-        base_currency: Money::Currency.new(from).iso_code,
-        converted_currency: Money::Currency.new(to).iso_code,
+    def find_rate(from:, to:, date:, cache: true)
+      result = find_by \
+        from_currency: from,
+        to_currency: to,
         date: date
+
+      result || fetch_rate_from_provider(from:, to:, date:, cache:)
     end
 
-    def find_rate_or_fetch(from:, to:, date:)
-      find_rate(from:, to:, date:) || fetch_rate_from_provider(from:, to:, date:)&.tap(&:save!)
-    end
+    def find_rates(from:, to:, start_date:, end_date: Date.current, cache: true)
+      rates = self.where(from_currency: from, to_currency: to, date: start_date..end_date).to_a
+      all_dates = (start_date..end_date).to_a.to_set
+      existing_dates = rates.map(&:date).to_set
+      missing_dates = all_dates - existing_dates
 
-    def get_rates(from, to, dates)
-      where(base_currency: from, converted_currency: to, date: dates).order(:date)
-    end
+      if missing_dates.any?
+        rates += fetch_rates_from_provider(from:, to:, dates: missing_dates, cache:)
+      end
 
-    def convert(value:, from:, to:, date:)
-      rate = ExchangeRate.find_by(base_currency: from, converted_currency: to, date:)
-      raise "Conversion from: #{from} to: #{to} on: #{date} not found" unless rate
-
-      value * rate.rate
+      rates
     end
   end
 end