fixed performance issue of ActiveRow [Closes #312]

Author: dg

src/Database/Table/Selection.php

@@ -989,6 +989,7 @@ public function getReferencingTable(string $table, ?string $column = null, $acti
 		if (!$prototype) {
 			$prototype = $this->createGroupedSelectionInstance($table, $column);
 			$prototype->where("$table.$column", array_keys((array) $this->rows));
+			$prototype->getSpecificCacheKey();
 		}
 
 		$clone = clone $prototype;

tests/Database/Explorer/Explorer.cache.observer.phpt

@@ -17,7 +17,7 @@ Nette\Database\Helpers::loadFromFile($connection, __DIR__ . "/../files/{$driverN
 
 $cacheStorage = Mockery::mock(Nette\Caching\Istorage::class);
 $cacheStorage->shouldReceive('read')->withAnyArgs()->once()->andReturn(['id' => true]);
-$cacheStorage->shouldReceive('read')->withAnyArgs()->times(4)->andReturn(['id' => true, 'author_id' => true]);
+$cacheStorage->shouldReceive('read')->withAnyArgs()->times(2)->andReturn(['id' => true, 'author_id' => true]);
 $cacheStorage->shouldReceive('write')->with(Mockery::any(), ['id' => true, 'author_id' => true, 'title' => true], []);
 
 $explorer = new Nette\Database\Explorer($connection, $structure, $conventions, $cacheStorage);