I need to model the following requirement in an Entity-Relationship diagram.
"Customers place orders, which contain a list of products with their quantity and prices. A shipment is made when products are ready to be delivered to a customer. A shipment may include products from multiple orders, but only if they all belong to the same customer."
The last sentence is the one I'm not sure about. I could have a Shipment entity in relationships with Product, Order and/or Customer and the cardinalities are easy to work out. But how can I express the fact that a Shipment concerns one and only one Customer, even though it may contain multiple orders? I couldn't find a way. Is it impossible?