Refactoring SqlCriteria, SqlModel. Add SqlResultProvider.fetchField().

model/SqlResultProvider.php

@@ -64,6 +64,11 @@ class SqlResultProvider
         return $this->result_items[$key];
     }
 
+    public function fetchField($field)
+    {
+        return current($this->result_items)->{$field};
+    }
+
     public function fetch()
     {
         return current($this->result_items);