与EF代码优先和静态表的多对多关系

Question

I'm using Entity Framework 5, and a code first approach.

I have a Product class, which can have zero-or-more ProductColors . The colors are prepopulated in the database using seeding. The color table should not be populated with new items using EF as it is a static list of items that will not grow. Colors are reused in many products.

My model classes:

public class Product
{
    public int ID { get; set; }
    public string Title { get; set; }
    public virtual ICollection<ProductColor> Colors { get; set; }
}

public class ProductColor
{
    public int ID { get; set; }
    public string Title { get; set; }
}

In my DbMigrationsConfiguration :

protected override void Seed(... context)
{
    context.ProductColors.AddOrUpdate(
        p => p.ID,
        new ProductColor(1, "White"),
        new ProductColor(2, "Black"),
        new ProductColor(3, "Red"));
}

In my DbContext :

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>().HasMany(x => x.Colors).WithMany();
}

public DbSet<Product> Products { get; set; }

My Products are created from a viewmodel object, both when they are created for the first time, and also later when they are edited:

Product product = new Product { ID = productViewModel.ID };

product.Colors = new List<ProductColor>();
foreach (int colorId in productViewModel.SelectedColorIds)
{
    ProductColor productColor = productColors.Find(m => m.ID == colorId);
    product.Colors.Add(productColor);
}

They are saved in the database like this when created:

db.Products.Add(product);
db.SaveChanges();

And like this when they are edited:

db.Entry(product).State = EntityState.Modified;
db.SaveChanges();

EF generates Products , ProductColor and ProductProductColor tables just fine initially. When the products are first created and saved, the colors are properly being saved in the ProductProductColor table.

But when I edit/modify the Product and Colors collection, the colors are not being updated in the database. Seems it doesn't recognize that the Colors collection has been modified. How can I make it so?

Sorry for the lengthy post, but I wanted to include all the elements in case someone needs the full picture.

Answer 1

I managed to find a solution. Instead of create a new Product instance (using the same ID as previously), I fetch the product from the database.

IQueryable<Product> products = 
    from p in db.Products.Include("Colors")
    select p;
Product product = Enumerable.FirstOrDefault(products, p => p.ID == id);

When I then change the Colors collection, it seems the tracking of items is working and serialized correctly into the database.

After some reading to understand why, I see fetching the product from the database creates a Proxy object that keeps track of the collection for me. When creating a Product manually, it is not able to track changes to collections.