MVC和Entity Framework中的MM关系仅在调试器模式下有效

Question

I am doing some project for school and have encountered a strange thing. Situation is like this:

In my SQL Server db I have 3 tables: Product, Ingredient and ProductIngredient. The last one is PJT between the first two.

I have this in a controller:

public ActionResult Index()
        {
            List<Product> products;
            using (FastfoodConnection conn = new FastfoodConnection())
            {
                products = conn.Products.ToList();

            }
            TestModel tM = new TestModel();
            tM.Products = products;
            return View(tM);
        }

and here is my view:

@foreach (var product in Model.Products)
{
    <tr>
        <td> @product.Name</td>
        <td> @product.Description</td>

    </tr>
    <tr>
        <th>Ingredients</th>
    <tr>
    <tr>
      <td>
        <ul>
           @foreach (var ingredient in product.Ingredients)
           {
            <li>@ingredient.IngredientName</li>
           }
        </ul>
      </td>
    </tr>
}
</table>

Now - when I set the breakpoint in controller and debug the code, step by step, everything works fine and result shows as I want it to. However, when I try to run the code (without debugging) I get ObjectDisposedException with description "The ObjectContext instance has been disposed and can no longer be used for operations that require a connection." Apparently this regards to product.Ingredients list which is null.

My question is: What could be the problem and how can it be solved?

Answer 1

You need to eager load the Ingredients . EF won't try to fetch those from the database until you need them, which is called Lazy Loading. Because you try to access them after you disposed your context, EF can't query the database for the Ingredients and you get an exception. Eager loading tells EF it should also fetch those entities in the same run when it gets the Products. It's always a good idea to use eager loading when you know beforehand you will need the entities. This avoids multiple db calls since everything gets fetched in one query. It can be invoked through the Include(...) method:

public ActionResult Index()
    {
        List<Product> products;
        using (FastfoodConnection conn = new FastfoodConnection())
        {
            products = conn.Products.Include(p => p.Ingredients).ToList();

        }
        TestModel tM = new TestModel();
        tM.Products = products;
        return View(tM);
    }

This should resolve your problem. Also make sure you have the collection of Ingredients marked as virtual in your product model, which enables lazy/eager loading, but since it works in debugging mode I guess that will be alright.