Problems with related objects in MVC 4

Question

I have setup a couple of objects:

Product.cs

namespace Print_Solutions.Models
{
    public class Product
    {
        public int ID              { get; set; }
        public string Manufacturer { get; set; }
        public string Model        { get; set; }
        public string PartNumber   { get; set; }
        public int ProductCategoryID      { get; set; }

        public virtual ICollection<ProductCategory> ProductCategory { get; set; }
    }
}

ProductCategory.cs

namespace Print_Solutions.Models
{
    public class ProductCategory
    {
        public int ID { get; set; }
        public string Name { get; set; }

        public virtual ICollection<Product> Products { get; set; }
    }
}

The DB Context

namespace Print_Solutions.DAL
{
    public class ApplicationContext : DbContext
    {
        public ApplicationContext() : base("DefaultConnection")
        {
        }

        public DbSet<ProductCategory> ProductCategories { get; set; }
        public DbSet<Product> Products { get; set; }
        public DbSet<ProductDetail> ProductDetails { get; set; }
        public DbSet<ProductDocument> ProductDocuments { get; set; }
        public DbSet<ProductImage> ProductImages { get; set; }
        public DbSet<RelatedProduct> RelatedProducts { get; set; }
    }
}

The Controller

namespace Print_Solutions.Controllers
{
    public class DashboardController : Controller
    {
        private ApplicationContext db = new ApplicationContext();

        public ActionResult Index()
        {
            ViewBag.Products = db.Products.ToList();
            ViewBag.ProductCategories = db.ProductCategories.ToList();
            return View();
        }
    }
}

The Problem

I have tried a couple of things in the view. Neither one of these seems to work. How can I access products from productcategories, and vice versa?

<ul>
    @foreach(Print_Solutions.Models.ProductCategory category in @ViewBag.ProductCategories)
    {
        <li>@category.Name
            <ul>
                @foreach(var product in @category.Products)
                {
                    <li>@product.Name</li>
                }
            </ul>
        </li>
    }
</ul>

or

<ul>
    @foreach (Print_Solutions.Models.Product product in @ViewBag.Products)
    {
        <li>@product.Name - @product.ProductCategory.Name</li>
    }
</ul>

I can access products, and product categories, but not through their related objects. What am I missing?

ProductCategory is a collection -- not a single instance.

Ian P
– Ian P

2014-06-26 18:09:47 +00:00
Commented Jun 26, 2014 at 18:09 — Ian P
– Ian P, Commented Jun 26, 2014 at 18:09
Constructor for ProductCategory at Product class ?

marathonman
– marathonman

2014-06-26 18:11:23 +00:00
Commented Jun 26, 2014 at 18:11 — marathonman
– marathonman, Commented Jun 26, 2014 at 18:11

Chris Pratt · Accepted Answer · 2014-06-26 18:25:37Z

Your problem in the first example is that you're using the @ in front of category.Products. That's going to raise a syntax error.

Your problem in the second example is that the inappropriately named ProductCategory property is actually a collection, not a single object. In other words, you can't access a property like Name directly off it. You would have to loop through this collection as well or otherwise join the values. For example, in this scenario, you'd probably just want to list all the categories the product belongs to, so you could do something like:

@string.Join(", ", product.ProductCategory.Select(m => m.Name))

Which creates an enumerable containing only the value for Name of each ProductCategory in the collection, and then joins them all together delineated by a comma and a space: Category1, Category2, Category3, ....

While I've got you, though, there's some important changes you should make:

Since these are related things, don't do two separate database queries (one for products and one for categories), instead, select one (probably the products based on your usage), and then join the other in the same query:
```
var products = db.Products.Include("ProductCategory").ToList();
```
Don't use ViewBag. Like ever. There's some times where its use may not be so bad, but until you can properly distinguish a good usage from a bad usage, it's better to just put a moratorium on the whole concept. The problem with ViewBag is that it uses dynamics; in other words, it is not evaluated at all at compile time, and either works or fails at runtime. The golden rule of software development is to always fail at compile. You want to know when you're building that something doesn't work, not when it's been deployed to your user days, weeks, months or even years after it's been developed. So, yeah, ViewBag is evil. Instead, use a strongly-typed view and pass your "model", in this case, your products into it:

Controller
```
var products = db.Products.Include("ProductCategory").ToList();
return View(products);
```
View
```
@model IEnumerable<Namespace.To.Product>

@foreach (var product in Model)
{
    ...
}
```

THanks for the tips!! I'll stop using ViewBag. (it was one of those things that felt wrong, but I couldn't figure out exactly what I needed to Google to get a better solution). If a Product will only ever belong to one category, is there a better way for me to define it where Product.Category.Name is possible? (assuming I rename that property)
Sure, just make it a simple reference property instead: public virtual ProductCategory ProductCategory { get; set; }
Final question! Why is it that I ProductCategory.Product.Model works, but Product.ProductCategory.Name throws a SQL exception where it cannot find dbo.ProductProductCategories. I know this is a naming convention issue, but I can't seem to figure out where I went wrong.
Initially, you had this set up as a many-to-many which means EF generated or at least expects to see a table called dbo.ProductProductCategories as a join table. If you don't have this table in your database, you probably have neglected to create a new migration / update your database since adding the relationship. However, since you've now indicated that many-to-many isn't what you wanted anyways, just change your code and you should be fine.

PWilliams0530 · Accepted Answer · 2014-06-26 18:21:29Z

Since Product Category is a collection, you may wish to loop thru the ProductCategory List (within the Product Object) within the product loop to print them off.

Alternately, you can create a View Model which contains only what you need. (Product Name, List of ProductCategoryNames) and pass that into things.

If you intend to do a post back, you can add ProductID to this view model (or CategoryID as well for that matter) and put them into a @Html.HiddenFor() so you can reference them later as needed.

Collectives™ on Stack Overflow

Problems with related objects in MVC 4

2 Answers 2

4 Comments

Comments

Hot Network Questions