Question

I have a problem with using EF 4.1 and many to many relationship. I decided to use database first approach (although I got the same problem with code first approach), and I created a model of the database:

I have a many to many relationship between Picture and Tag tables. In the DB it is created as a new table called “PictureTag” with fields “Picture_Id” and “Tag_Id”.

I am adding data to my database with the following piece of code:

if (tags != null)
{
    foreach (HtmlNode tagNode in tags)
    {
        string tagString = tagNode.InnerText.Remove(0, 1);
        Tag tag;
        if (db.Tags.Any(q => q.TagName == tagString))
        {
            tag = db.Tags.Single(q => q.TagName == tagString);
        }
        else
        {
            tag = new Tag { TagName = tagString };
        }
        pic.Tags.Add(tag);
    }
}
List<DbEntityValidationResult> list = db.GetValidationErrors().ToList();
db.Pictures.Add(pic);
db.SaveChanges();

This is of course just a part of my code. It is a part of my Crawler function. The code works fine, doesn’t generate any errors, and works on a single DB context.

After this function is done I am able to see data in my DB, all three pictures, tags and entities in PictureTag table.

Yet when I try to put on my view pictures, and tags that correspond to that picture, than I can’t see any tags. Like none of the pictures had any tags attached to them. I know it is not true, because I see data in my DB, so I don’t know what is wrong…

This is the code fr my view:

@model IEnumerable<SikSolution.Picture>

@{
    ViewBag.Title = "Index";
}

<h2>Index</h2>
@foreach (var item in Model)
{

    <div>
        <img src="@item.Link" alt="@item.Name" />
        <br />
        <span>
            Tags: @foreach (var tag in item.Tags)
                  {
                      Html.Label(tag.TagName + ", ");
                  }
        </span>
    </div> 
}

I really don’t know what is wrong… I would appreciate any advice or pointing me in the right direction.

Thanks!

Answer 1

Possible explanation is that you don’t use lazy loading and also didn’t load your Picture collection including the related tags (eager loading). It should look like this:

//...
var pictures = db.Pictures.Include("Tags") // or Include(p => p.Tags) in EF 4.1
    .Where(...)
    .ToList();

return View(pictures);