asp.net - Entity Framework Updating with Related Entity

Question

I'm using the EF to try to update an entity with ASP.NET. I'm creating an entity, setting it's properties then passing it back to the EF on a separate layer with the ID so the change can be applied. I'm doing this because I only store the ID of the entity when it's been bound to the UI controls.

Everything works for standard properties, but I can't update the Category.ID of a Product (a related entity). I've tried EntityKey, EntityReference and a few other but the category ID isn't saved. This is what I have:

Product product = new Product();
product.CategoryReference.EntityKey = new EntityKey("ShopEntities.Categories", "CategoryID", categoryId);
product.Name = txtName.Text.Trim();
... other properties
StockControlDAL.EditProduct(productId, product);

public static void EditProduct(int productId, Product product) {
 using(var context = new ShopEntities()) {
     var key = new EntityKey("ShopEntities.Products", "ProductID", productId);
     context.Attach(new Product() { ProductID = productId, EntityKey = key });
     context.AcceptAllChanges();
     product.EntityKey = key;
     product.ProductID = productId;
     context.ApplyPropertyChanges("ShopEntities.Products", product);
     context.SaveChanges();
 }
}

I really want to use the EF but I seem to be having a few problems with using it with ASP.NET.

Answer 1

The reason this fails is two fold.

1. In order to update a Reference (i.e. Product.Category) you have to have the original reference value in the context too.
2. ApplyPropertyChanges(...) only applies to regular / scalar properties of the Entity, the reference is left unchanged

So I would do something like this (Note this code makes heavy use of a trick called stub entities to avoid mucking around with EntityKeys)

Product product = new Product();
// Use a stub because it is much easier.
product.Category = new Category {CategoryID = selectedCategoryID};
product.Name = txtName.Text.Trim();
... other properties

StockControlDAL.EditProduct(productId, originalCategoryID);


public static void EditProduct(Product product, int originalCategoryID ) {
 using(var context = new ShopEntities()) 
 {
     // Attach a stub entity (and stub related entity)
     var databaseProduct = new Product { 
             ProductID = product.ProductID, 
             Category = new Category {CategoryID = originalCategoryID}
         };
     context.AttachTo("Products", databaseProduct);

     // Okay everything is now in the original state
     // NOTE: No need to call AcceptAllChanges() etc, because 
     // Attach puts things into ObjectContext in the unchanged state

     // Copy the scalar properties across from updated product 
     // into databaseProduct in the ObjectContext
     context.ApplyPropertyChanges("ShopEntities.Products", product);

     // Need to attach the updated Category and modify the 
     // databaseProduct.Category but only if the Category has changed. 
     // Again using a stub.
     if (databaseProduct.Category.CategoryID != product.Category.CategoryID)
     {
         var newlySelectedCategory = 
                 new Category {
                     CategoryID = product.Category.CategoryID
                 };

         context.AttachTo("Categories", newlySelectedCategory)

         databaseProduct.Category = newlySelectedCategory;

     }

     context.SaveChanges();
 }
}

This will do the job, assuming no typos etc.