BIG Kuddos to Julien for spotting this bug and suggesting the solution... I'm...

BIG Kuddos to Julien for spotting this bug and suggesting the solution... I'm thinking this could have easily slipped by!  

git-svn-id: http://dev.piwik.org/svn/trunk@6646 59fd770c-687e-43c8-a1e3-f5a4ff64c105

core/DataTable/Row.php

@@ -84,7 +84,20 @@ class Piwik_DataTable_Row
 			&& $row[self::DATATABLE_ASSOCIATED] instanceof Piwik_DataTable)
 		{
 			$this->setSubtable($row[self::DATATABLE_ASSOCIATED]);
-		}	
+		}
+	}
+	
+	/**
+	 * Because $this->c[self::DATATABLE_ASSOCIATED] is negative when the table is in memory,
+	 * we must prior to serialize() call, make sure the ID is saved as positive integer
+	 */
+	public function __sleep()
+	{
+		if(!empty($this->c[self::DATATABLE_ASSOCIATED]) && $this->c[self::DATATABLE_ASSOCIATED] < 0)
+		{
+			$this->c[self::DATATABLE_ASSOCIATED] = abs($this->c[self::DATATABLE_ASSOCIATED]);
+		}
+		return array('c');
 	}
 	
 	/**