Please find my expected result below

But I am getting result as below

where I have choosed Order Id from orders table and Product Id from Order Items tables and I have used below measure order_to_orderitem = CALCULATE(SUM('Order Items'[Unit Price]),USERELATIONSHIP(orders[key12],'Order Items'[key12]))
Note: key12 is merged column of Order Id and Product Id in both orders and Order Items table
I have a datamodel as shown below

I have given a relationship as shown below

It has been created as shown below where From Table is Order Items and To Table is orders whereas while creating I have given orders as From Table and Order Items as To Table

Orders table:
Order Items table:


