Asp.Net MVC 6 Entity Framework - Model relationships

Your product class should be something like below

public class Product
{
public int ProductID { get; set; }
public string ProductName { get; set; }
public decimal ProductPrice { get; set; }

[ForeignKey("Category")]
public int CategoryID {get;set;}
public virtual Category category { get; set; }
}

Now you will have a column in your Product table which would store which category it would belong to.

To get a dropdown with all available Categories to enable selection , you could try the following

 ViewBag.CategoryID= new SelectList(db.Category, "CategoryID", "CategoryName");

And in the View you could use it as follows

@Html.DropDownList("CategoryID", null, htmlAttributes: new { @class = "form-control" })

Product needs to contain CategoryId (foreign key). Then you can tell EF how relationship looks like (even though EF will probably figure that out by itself if namings are done correct):

public class Category
{
    public int CategoryId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Product> Products { get; set; } 
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public int CategoryId { get; set; }
    public virtual Category Category { get; set; }
}

public class StoreContext : DbContext  
{ 
    public DbSet<Category> Categories{ get; set; } 
    public DbSet<Product> Products { get; set; } 
    protected override void OnModelCreating(DbModelBuilder modelBuilder) 
    { 
        // optionally, even EF can follow convention and detect 1-m relationship between product and category. 
        modelBuilder.Entity<Product>().HasRequired(p => p.Category) 
            .WithMany(c => c.Products) 
            .HasForeignKey(p => p.CategoryId);
    }
}

When creating a new product, set CategoryId property with correct value, that will connect it to the category.

Your ViewModel should look something like this (can contain whole Product class or individual properties that you have to map later):

public class ProductViewModel
{
    public ProductViewModel() 
    { 
        EditableProduct = new Product();
    }
    public List<SelectListItem> Categories { get; set; }
    public Product EditableProduct { get; set; }
}

And ProductController will fill all the categories and prepare the view model:

public ActionResult Edit(int? id)
{
    var model = new ProductViewModel();
    model.Categories = dbContext.Categories.Select(x => new SelectListItem() {Text = x.Name, Value = x.Id.ToString()}).ToList();

    // check if product even exists
    model.EditableProduct = id.HasValue ? dbContext.Products.Find(id.Value) : new Product();

    return View(model);
}

[HttpPost, ValidateAntiForgeryToken]
public ActionResult Edit(ProductViewModel model)
{
    // check validation
    Product product;
    if (model.EditableProduct.Id > 0)
        product = dbContext.Products.Find(model.EditableProduct.Id);
    else
    {
        product = new Product();
        dbContext.Products.Add(product);
    }

    product.CategoryId = model.EditableProduct.CategoryId;
    product.Name = model.EditableProduct.Name;

    // add try/catch 
    dbContext.SaveChanges();

    return RedirectToAction("Edit", new {id = product.Id});
}

Razor view can then render categories:

@Html.DropDownListFor(x => x.EditableProduct.CategoryId, Model.Categories)

This is the general pattern you can follow with all look-up lists and one-to-many relationships.

Word of advice: don't "leak" your database/domain model into view. Put name, id, etc properties to View Model class, and map them into EF model when request is received and validation is done on them. Try to avoid putting Product object into view model, like I did in this example!

This seems to be a bug in MVC Scaffolding, you can find more details here: https://github.com/aspnet/Scaffolding/issues/149

Try something like this in your view:

@{ var categories = (IEnumerable<SelectListItem>)ViewData["CategoryID"]; }
<select asp-for="CategoryID" asp-items="categories" class="form-control"></select>

This solved similar issue for me.