refactored MongoModel to hide fetch method, fixed PHPDoc errors

model/DbStatement.php

@@ -66,6 +66,7 @@ abstract class DbStatement
 
     /**
      * @param string $field
+     * @return array Value of queried field for all matching rows
      */
     public function fetchColumn($field)
     {
@@ -78,6 +79,7 @@ abstract class DbStatement
 
     /**
      * @param string $field
+     * @return mixed Value of queried filed for first matching row
      */
     public function fetchField($field)
     {
@@ -108,6 +110,7 @@ abstract class DbStatement
     abstract public function numRows();
 
     /**
+     * @param mixed $request
      * @return bool
      */
     abstract protected function driverExecute($request);