RVRNEXT-5 add model and repository for currency exchange rate

src/Repository/CurrencyExchangeRateRepository.php

@@ -0,0 +1,24 @@
+<?php namespace RVR\Repository;
+
+use Generator;
+use Container;
+use RVR\PersistentData\Model\Currency;
+use RVR\PersistentData\Model\CurrencyExchangeRate;
+use SokoWeb\Database\Query\Select;
+
+class CurrencyExchangeRateRepository
+{
+    public function getById(int $id): ?CurrencyExchangeRate
+    {
+        return \Container::$persistentDataManager->selectFromDbById($id, CurrencyExchangeRate::class);
+    }
+
+    public function getAllByCurrency(Currency $currency): Generator
+    {
+        $select = new Select(Container::$dbConnection);
+        $select->where('currency_id', '=', $currency->getId());
+        $select->orderBy('valid_from');
+
+        yield from Container::$persistentDataManager->selectMultipleFromDb($select, CurrencyExchangeRate::class);
+    }
+}