add group by clause

src/Storm/Model/PersistenceStrategy/Db.php

@@ -196,6 +196,11 @@ class Storm_Model_PersistenceStrategy_Db
         continue;
       }
 
+      if ('group_by' === $field) {
+        $select->group($value);
+        continue;
+      }
+
       $select->where($this->_generateWhereClauseForKeyAndValue($field, $value));
     }