实体框架savechanges()不工作EF5， . net 4

我有以下代码来更新一些库存值…

private static void UpdateInventory(int prodId, int qty)
    {
        using (var context = new uStore7_1Entities())
        {
            //Get the catalogNo and ProductUnitID of the product passed in so we can find all identical products that might just be boxed differently
            var currProdItem = context.Products.Where(c => c.ProductID.Equals(prodId))
                                       .Select(c => new {c.CatalogNo, c.ProductUnitID}).FirstOrDefault();
            //Get the units per box factor for calculating total product ordered
            var prodIdAmount =
                context.ProductUnits.Where(pa => pa.ProductUnitID.Equals(currProdItem.ProductUnitID))
                       .Select(pa => pa.Amount)
                       .FirstOrDefault();
            //Calculate the total number of units for this item
            var prodUnits = qty*prodIdAmount;
            //Get the entire list of products with the specified catalog number excluding the product passed in
            var uStoreProducts =
                context.Products.Where(p => p.CatalogNo.Equals(currProdItem.CatalogNo) && !p.ProductID.Equals(prodId))
                       .Select(p => p.ProductID);

            //Loop through each product in the uStoreProductsList
            foreach (var uStoreProduct in uStoreProducts)
            {
                var currentProduct = uStoreProduct;
                //Get the current product's ProductUnitId to get the 'pieces' per "box"
                var currentUnitId =
                    context.Products.Where(u => u.ProductID.Equals(currentProduct))
                           .Select(u => u.ProductUnitID)
                           .FirstOrDefault();
                //Use the ProductUnitId to get the "Amount" from the ProductUnits table.
                var inventoryFactor =
                    context.ProductUnits.Where(i => i.ProductUnitID.Equals(currentUnitId))
                           .Select(i => i.Amount)
                           .FirstOrDefault();
                //Divide the quantity passed 
                var qtyInUnits = prodUnits/inventoryFactor;
                var inventory =
                    context.ProductInventories.Where(pi => pi.ProductID.Equals(currentProduct))
                           .Select(pi => pi.InventoryQuantity)
                           .FirstOrDefault();
                /*var inventory = (from i in context.ProductInventories
                                where i.ProductID == currentProduct
                                select i).FirstOrDefault();
                */

                if (inventory != null)
                {
                    var newinv = inventory - qtyInUnits;
                    inventory = newinv;
                    //context.SaveChanges();
                }
            }
            context.SaveChanges();
        }
    }

SaveChanges()似乎没有更新任何东西。我已经调试了它，库存值被更改为所需的值，但由于某种原因，它没有更新。我在循环内和循环外都试过了，都没有变化。什么好主意吗?我遗漏了什么?