Rate exchange fix (#377)

* FIX cache hit issue

* Change to proper method

* FIX cache is read heavy, so change to find

app/models/exchange_rate/provided.rb

@@ -19,12 +19,23 @@ module ExchangeRate::Provided
       return nil unless response.success? # Provider error
 
       rate = response.data
-      ExchangeRate.find_or_create_by!(
-        from_currency: rate.from,
-        to_currency: rate.to,
-        date: rate.date,
-        rate: rate.rate
-      ) if cache
+      begin
+        ExchangeRate.find_or_create_by!(
+          from_currency: rate.from,
+          to_currency: rate.to,
+          date: rate.date
+        ) do |exchange_rate|
+          exchange_rate.rate = rate.rate
+        end if cache
+      rescue ActiveRecord::RecordNotUnique
+        # Race condition: another process inserted between our SELECT and INSERT
+        # Retry by finding the existing record
+        ExchangeRate.find_by!(
+          from_currency: rate.from,
+          to_currency: rate.to,
+          date: rate.date
+        ) if cache
+      end
       rate
     end