Modeling a many-to-many relationship in ASP.NET MVC using LINQ to SQL

Question

I've been reading and watching videos about MVC and I'm starting to get a little confused. The simple tutorials are far too simple and they seem to only involve a single database table. Whoopty doo!

The advanced tutorials assume a lot of knowledge is present and I have trouble following those. Not to mention, there are 15 ways to model your database and nobody wants to explain why they do it this way or that way.

So, I'm looking for a simple tutorial or explanation of what process you would go through to design a simple CRUD application that involves a many-to-many relationship, explained below. I may not have provided enough information so feel free to request updates.

Updates: I would be interested in seeing a Linq To Sql solution.

I went through the nerddinner PDF and my model has to be a little different than his. I want a many-to-many relationship with my Meal and my Ingredients. He just uses a 1-to-many with his Dinner and RSVPs. Also, he never shows how he attached the RSVPs to his Dinner model. This is essentially the problem I'm having. Why is it that my Meal model does not contain a list of Ingredients? I have the proper foreign keys in place. I'm not sure where or how I would set it up so that I can access the ingredients through the Meal model if I wanted to print them on the details view or something.

Meals

• Id
• Title
• Description

Ingredients

• Id
• Name

Meals-Ingredients

• Id_Meal
• Id_Ingredient

Answer 1

I have many examples of this type of relationship in my current project. I'm using MVC 1 and LINQ-to-SQL. I went through exactly the same frustration then as you are experiencing now. The biggest hurdle is accepting the fact that LINQ-to-SQL doesn't directly manage many-to-many relationships, but understanding that it doesn't need to in order to get the information you require.

Let's start with the R in CRUD, since it's the easiest way to demonstrate what needs to be done. Now, at this point I would recommend creating a strongly-typed view model, just to ease the task of aggregating your view data and to simplify the assignment of the meal data to the Details view:

public class MealDetailsViewModel
{
    public int Id_Meal { get; set; }
    public string Title { get; set; }
    public string Description { get; set; }

    private List<Ingredient> _Ingredients = new List<Ingredient>();
    public List<Ingredient> Ingredients
    {
       get { return _Ingredients; }
       set { _Ingredients = value; }
    }
}

To retrieve a meal and its list of ingredients, here's the controller method:

public ActionResult Details (int mealID)
{
    Meal result = DataContext.Meals
        .Where(a => a.Id_Meal == mealID)
        .SingleOrDefault();

    MealDetailsViewModel viewModel = new MealDetailsViewModel
    {
        Id_Meal = result.Id,
        Title = result.Title,
        Description = result.Description,
        Ingredients = result.Meals-Ingredients
            .Where(a => a.Id_Meal == mealID)
            .Select(a => a.Ingredient)
            .ToList()
    };

    return View(viewModel);
}

As previously stated, LINQ-to-SQL doesn't directly support many-to-many relationships, which is why you cannot see the Ingredients entity directly from Meal. However, as illustrated in the controller action method, you can access the association entity (Meals-Ingredients) from the Meal entity. From the association entity, you can access the Ingredients entity to get the list of ingredients. The view would look something like this:

<%@ Page Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="ViewPage<MealDetailsViewModel>" %>

<asp:Content ID="mainMealDetails" ContentPlaceHolderID="MainContent" runat="server">
    <h2><%= Model.Title %></h2>
    <h3><%= Model.Description %></h3>
    <br />
    <p>Ingredients:</p>
    <ul>
        <% foreach(Ingredient item in Model.Ingredients) 
           { %>
            <li><%= item.Name %></li>
        <% } %>
    </ul>
    <br />
    <%= Html.ActionLink("Update", "Edit", new { id = Model.Meal_ID }) %> |
    <%= Html.ActionLink("Add Ingredient", "IngredientCreate", new{ id = Model.Meal_ID }) %> |
    <%= Html.ActionLink("Delete", "Delete", new { id = Model.Meal_ID }) %> |
    <%= Html.ActionLink("Back to Menu List", "Index") %>
</asp.Content>

If your database schema is correctly set up with the foreign key relationships you require, this approach should give you the outcome you're looking for.

Answer 2

I highly recommend Steve Sanderson's 'Pro ASP.NET MVC Framework'. It's by far the best ASP.NET MVC guide I've seen, and in my opinion much better than the Wrox book by the MSoft team. Well worth the money if you want to get into it.

Amazon Link