Purchase Order Contracts - link multiple purchase orders together into a contract

The purchasing manager/procurement user will agree a contract with a supplier to supply stock over a period say between 6 and 12 months. The stock will called off over this time.Delivery Dates will be forecasted. One line for the stock item with multiple delivery dates does not work with MRP systems. What is required is separate purchase orders linked together as a contract. The contract will have a contract number.