Question

I’ve created three tables – Podcast, Category and CategoryLink.
The CategoryLink table contains only two columns – PodcastId and CategoryId.
I have described many-to-many relations between these tables as written in this article.
All works fine but I don’t know how to make query correctly.
Now I do this by the following way:

var ps = db.Podcasts.Where(p => p.Status.SysStatus > 0);
if (category_id.HasValue)
    ps = ps.Where(p => p.Categories.Where(c => c.Id == category_id.Value).FirstOrDefault() != null);

It works, but I think it’s a little dirty way 🙂

It give me the following SQL query (I’ve removed excess columns and strings):

    SELECT 
[Project2].[Id] AS [Id]
FROM ( SELECT 
    [Extent1].[Id] AS [Id], 
    (SELECT TOP (1) 
        [Extent3].[CategoryId] AS [CategoryId]
        FROM [dbo].[CategoryLink] AS [Extent3]
        WHERE ([Extent1].[Id] = [Extent3].[PodcastId]) AND ([Extent3].[CategoryId] = 1)) AS [C1]
    FROM  [dbo].[Podcast] AS [Extent1]
    INNER JOIN [dbo].[PodcastStatus] AS [Extent2] ON [Extent1].[StatusId] = [Extent2].[Id]
    WHERE [Extent2].[SysStatus] > 0
)  AS [Project2]
WHERE [Project2].[C1] IS NOT NULL

The querstion is that how to do query so I can get a ‘normal’ inner join without sub-queries?

Thanks.

Answer 1

I am not sure what you’re trying to accomplish. Many-to-Many is not much different from Many-to-One in the object mapping, except that you can access the relationship from both sides:

using System.Data.Entity; // For .Include Extension

// Eager-load Categories
var podcasts = db.Prodcasts.Include(x => x.Categories);
foreach (var p in podcats)
{
    foreach (var c in p.Categories)
    {
    }
}

var categories = db.Categories.Include(x => x.Podcasts);
foreach (var c in categories)
{
    foreach (var p in c.Podcasts)
    {
    }
 }

Depending on what you try to accomplish you might also want to do something like this:

from c in Customers
join p in Purchases on c.ID equals p.CustomerID           // first join
join pi in PurchaseItems on p.ID equals pi.PurchaseID     // second join
select new
{
c.Name, p.Description, pi.Detail
}

If you’re new to LINQ I recommend taking a look at LinqPad. It includes quite a few examples that should give you an idea on how it works.

Update:

Could you maybe elaborate what you actually want to get?

The select in my second example can also be done without a PurchaseItems class:

var q = (from p in db.Prodcasts
        from c in p.Categories // note that it is using "p" instead of db.Categories
        select new 
        {
            Podcast = p.Name,
            CategoryName = c.Name
        }).ToList();