สอบถาม Fatal error: Call to a member function query() on a non-object

<?PHP

$str = new class_name(); //ชื่อ class 

$str = "SELECT tb_Center.CenterNo, tb_Function.Function_Name, tb_Contract.Contract_No,tb_Equipment_Type.Equipment_Type, tb_Purchase.Budget_New,
tb_Purchase.Budget_Replace, tb_Equipment.Budget_Price
FROM (tb_Function INNER JOIN (tb_Center_Function INNER JOIN tb_Center ON tb_Center_Function.ID_Center = tb_Center.ID_Center) 
ON tb_Function.ID_Function = tb_Center_Function.ID_Function) 
INNER JOIN ((tb_Equipment_Type INNER JOIN (tb_Contract INNER JOIN tb_Equipment ON tb_Contract.ID_Contract = tb_Equipment.ID_Contract)
ON tb_Equipment_Type.ID_Equipment_Type = tb_Equipment.ID_Equipment_Type) INNER JOIN tb_Purchase ON tb_Equipment.ID_Equipment = tb_Purchase.ID_Equipment) 
ON tb_Function.ID_Function = tb_Purchase.ID_Function 
 where tb_Contract.ID_Contract  = ".$selcontract;

		$result = $db->query($str);
		while ($row=$result->fetch()){
				
				 $CenterNo= $row['CenterNo'];
				 $Contract_No= $row['Contract_No'];
				 $Functionname= $row['Function_Name'];
				 $Equipment_Type= $row['Equipment_Type'];
				 $Budget_New = $row['Budget_New'];
				 $Budget_Replace=$row['Budget_Replace'];
				 $Budget_Price= $row['Budget_Price'];
				 $RentMonth = $Budget_Price*($Budget_New+$Budget_Replace);
				 $RentYear = $RentMonth*12;
				 $RentYear3 = $RentYear*3;
		}
}
	
		  ?>