New function for grouping elements by unique key

This func have the same behaviour as `as_array` function, but the last one make array of [keys => arrays].

`groupByUniqueKey` just an array of [unique key => array].

~dev_rating/modules/database/classes/Kohana/Database/Result.php

@@ -171,6 +171,18 @@ abstract class Kohana_Database_Result implements Countable, Iterator, SeekableIt
 		return $results;
 	}
 
+    public function groupByUniqueKey($key) {
+        $results = [];
+
+        foreach ($this as $row) {
+            $results[$row[$key]] = $row;
+        }
+
+        $this->rewind();
+
+        return $results;
+    }
+
 	/**
 	 * Return the named column from the current row.
 	 *