Question

I have two tables: categories and articles. One category can have many articles. I want to display overview of new articles separated by categories. But I want to display only first five articles per each category. How should I write query to solve this?

My first idea was something like this:

var cat = from c in ctx.categories
          where c.IsPublic == true
          select c;

But this contains all articles and I’m not sure how to write something like c.articles.Take(5) to query. Is it possible?

Oh, and it is ASP.NET MVC 2 with Entity Framework.

Answer 1

This worked for me. Important here is to switch off lazy loading to avoid that all articles are loaded when you iterate through the collection:

ctx.ContextOptions.LazyLoadingEnabled = false; // assuming you are in EF 4.0

var query = ctx.categories
    .Where(c => c.IsPublic)
    .Select(c => new { 
        Category = c, 
        Articles = c.Articles.Take(5)
    }).ToList().Select(x => x.Category);

foreach (var category in query)
{
    foreach (var article in category.Articles)
    {
        // runs only 5 times through this loop
    }
}

Edit

If you don’t dispose ctx and work with the context more after the code snippet above you probably better reenable lazy loading again …

ctx.ContextOptions.LazyLoadingEnabled = true;

… before you get into trouble with other queries if your application mostly relies on lazy loading.