Question

I am trying to create a database with a many-to-many relationship using EF code first.

public class Item
{
    public int ItemId { get; set; }
    public String Description { get; set; }
    public ICollection<Tag> Tags { get; set; }

    public Item()
    {
        Tags = new HashSet<Tag>();
    }
}

public class Tag
{
    public int TagId { get; set; }
    public String Text { get; set; }
    public ICollection<Item> Presentations { get; set; }

    public Tag()
    {
        Presentations = new HashSet<Item>();
    }
}

public class ItemsEntities : DbContext
{
    public DbSet<Item> Items { get; set; }
    public DbSet<Tag> Tags { get; set; }
}

After that I’m adding an Item to the database

var tag = new Tag { Text = "tag1" };
var item = new Item
{
    Description = "description1",
    Tags = new List<Tag>()
};
item.Tags.Add(tag);
using (var db = new ItemsEntities())
{
    db.Items.Add(item);
    db.SaveChanges();
}

The problem is that I can’t output items with their associated tags. The controller looks like this:

public ActionResult Index()
{
    ItemsEntities db = new ItemsEntities();
    return View(db.Items.ToList());
}

and the view page has the following code:

@foreach (var item in Model) 
{
   <tr>
    <td>
        @Html.DisplayFor(model => item.Description)
    </td>
    <td>
        @foreach (var tag in item.Tags)
        {
            @tag.Text
        }
    </td>
</tr>
}

I expect the table to contain “description1” and “tag1” but I get only “description1”. I really don’t understand where the problem is. What is the correct way to do this?

Answer 1

Your navigation properties need to be marked virtual.

public class Item
{
    public int ItemId { get; set; }
    public String Description { get; set; }
    public virtual ICollection<Tag> Tags { get; set; }

    public Item()
    {
        Tags = new HashSet<Tag>();
    }
}

public class Tag
{
    public int TagId { get; set; }
    public String Text { get; set; }
    public virtual ICollection<Item> Presentations { get; set; }

    public Tag()
    {
        Presentations = new HashSet<Item>();
    }
}