Various fixes.

app/Support/Http/Api/ExchangeRateConverter.php

@@ -25,14 +25,16 @@ namespace FireflyIII\Support\Http\Api;
 
 use Carbon\Carbon;
 use FireflyIII\Exceptions\FireflyException;
+use FireflyIII\Models\CurrencyExchangeRate;
 use FireflyIII\Models\TransactionCurrency;
+use FireflyIII\Support\CacheProperties;
 
 /**
  * Class ExchangeRateConverter
  */
 class ExchangeRateConverter
 {
-    use ConvertsExchangeRates;
+    //use ConvertsExchangeRates;
 
     /**
      * @param TransactionCurrency $from
@@ -59,17 +61,138 @@ class ExchangeRateConverter
      */
     public function getCurrencyRate(TransactionCurrency $from, TransactionCurrency $to, Carbon $date): string
     {
-        if (null === $this->enabled) {
-            $this->getPreference();
-        }
-
-        // if not enabled, return "1"
-        if (false === $this->enabled) {
-            return '1';
-        }
-
         $rate = $this->getRate($from, $to, $date);
         return '0' === $rate ? '1' : $rate;
     }
 
+    /**
+     * @param TransactionCurrency $from
+     * @param TransactionCurrency $to
+     * @param Carbon              $date
+     *
+     * @return string
+     * @throws FireflyException
+     */
+    private function getRate(TransactionCurrency $from, TransactionCurrency $to, Carbon $date): string
+    {
+        // first attempt:
+        $rate = $this->getFromDB((int)$from->id, (int)$to->id, $date->format('Y-m-d'));
+        if (null !== $rate) {
+            return $rate;
+        }
+        // no result. perhaps the other way around?
+        $rate = $this->getFromDB((int)$to->id, (int)$from->id, $date->format('Y-m-d'));
+        if (null !== $rate) {
+            return bcdiv('1', $rate);
+        }
+
+        // if nothing in place, fall back on the rate for $from to EUR
+        $first  = $this->getEuroRate($from, $date);
+        $second = $this->getEuroRate($to, $date);
+
+        // combined (if present), they can be used to calculate the necessary conversion rate.
+        if ('0' === $first || '0' === $second) {
+            return '0';
+        }
+
+        $second = bcdiv('1', $second);
+        return bcmul($first, $second);
+    }
+
+    /**
+     * @param int    $from
+     * @param int    $to
+     * @param string $date
+     *
+     * @return string|null
+     */
+    private function getFromDB(int $from, int $to, string $date): ?string
+    {
+        $key = sprintf('cer-%d-%d-%s', $from, $to, $date);
+
+        $cache = new CacheProperties();
+        $cache->addProperty($key);
+        if ($cache->has()) {
+            $rate = $cache->get();
+            if ('' === $rate) {
+                return null;
+            }
+            return $rate;
+        }
+        app('log')->debug(sprintf('Going to get rate #%d->#%d (%s) from DB.', $from, $to, $date));
+
+        /** @var CurrencyExchangeRate $result */
+        $result = auth()->user()
+                        ->currencyExchangeRates()
+                        ->where('from_currency_id', $from)
+                        ->where('to_currency_id', $to)
+                        ->where('date', '<=', $date)
+                        ->orderBy('date', 'DESC')
+                        ->first();
+        $rate   = (string)$result?->rate;
+        $cache->store($rate);
+        if ('' === $rate) {
+            return null;
+        }
+        return $rate;
+    }
+
+
+    /**
+     * @param TransactionCurrency $currency
+     * @param Carbon              $date
+     *
+     * @return string
+     * @throws FireflyException
+     *
+     */
+    private function getEuroRate(TransactionCurrency $currency, Carbon $date): string
+    {
+        $euroId = $this->getEuroId();
+        if ($euroId === (int)$currency->id) {
+            return '1';
+        }
+        $rate = $this->getFromDB((int)$currency->id, $euroId, $date->format('Y-m-d'));
+
+        if (null !== $rate) {
+            //            app('log')->debug(sprintf('Rate for %s to EUR is %s.', $currency->code, $rate));
+            return $rate;
+        }
+        $rate = $this->getFromDB($euroId, (int)$currency->id, $date->format('Y-m-d'));
+        if (null !== $rate) {
+            return bcdiv('1', $rate);
+            //            app('log')->debug(sprintf('Inverted rate for %s to EUR is %s.', $currency->code, $rate));
+            //return $rate;
+        }
+        // grab backup values from config file:
+        $backup = config(sprintf('cer.rates.%s', $currency->code));
+        if (null !== $backup) {
+            return bcdiv('1', (string)$backup);
+            // app('log')->debug(sprintf('Backup rate for %s to EUR is %s.', $currency->code, $backup));
+            //return $backup;
+        }
+
+        //        app('log')->debug(sprintf('No rate for %s to EUR.', $currency->code));
+        return '0';
+    }
+
+
+    /**
+     * @return int
+     * @throws FireflyException
+     */
+    private function getEuroId(): int
+    {
+        $cache = new CacheProperties();
+        $cache->addProperty('cer-euro-id');
+        if ($cache->has()) {
+            return (int)$cache->get();
+        }
+        $euro = TransactionCurrency::whereCode('EUR')->first();
+        if (null === $euro) {
+            throw new FireflyException('Cannot find EUR in system, cannot do currency conversion.');
+        }
+        $cache->store((int)$euro->id);
+        return (int)$euro->id;
+    }
 }