Question

I couldn’t find or come up with a generic and elegant algorithm that would let me populate the tree-like structure.
The simplest example is a blog archive: I have a bunch of records which can be selected and sorted by date.
I want to have a tree where years may be top level, months are next level and actual post titles are next again.

So far I’ve came up with a naive straight forward implementation that works, but I’m sure that could be improved using LINQ, etc.
Here I just sort records by date and iterate checking if the year or month has changed, and add tree nodes accordingly.
“BlogEntry” is a class that has a reference to both a parent and children and is later used to generate HTML.

I welcome suggestions on improving the algorithm!

IEnumerable<Post> posts = db.Posts.OrderBy(p => p.DateCreated);
var topPost = posts.First();

int curYear = topPost.DateCreated.Year;
int curMonth = topPost.DateCreated.Month;

//create first "year-level" item
var topYear = new BlogEntry { Name = topPost.DateCreated.Year.ToString().ToLink(string.Empty) };
entries.Add(topYear);
var currentYear = topYear;

var topMonth = new BlogEntry { Name = topPost.DateCreated.ToString("MMMM").ToLink(string.Empty), Parent = currentYear };
currentYear.Children.Add(topMonth);
var currentMonth = topMonth;

foreach (var post in posts)
{
    if(post.DateCreated.Year == curYear)
    {
        if (post.DateCreated.Month != curMonth)
        {
            //create "month-level" item
            var month = new BlogEntry { Name = post.DateCreated.ToString("MMMM").ToLink(string.Empty), Parent = currentYear };
            currentYear.Children.Add(month);
            currentMonth = month;

            curMonth = post.DateCreated.Month;
        }

        //create "blog entry level" item
        var blogEntry = new BlogEntry { Name = post.Title.ToLink("/Post/" + post.PostID + "/" + post.Title.ToSeoUrl() ), Parent = currentMonth };
        currentMonth.Children.Add(blogEntry);
    }
    else
    {
        //create "year-level" item
        var year = new BlogEntry { Name = post.DateCreated.Year.ToString().ToLink(string.Empty) };
        entries.Add(year);
        currentYear = year;

        curMonth = post.DateCreated.Month;
        curYear = post.DateCreated.Year;
    }
}

Answer 1

I’ve created a test example, to check the correctness of the logic. I think this is what you need.

public class BlogEntyTreeItem
{
   public string Text { set; get; }
   public string URL { set; get; }
   public List<BlogEntyTreeItem> Children { set; get; }

   public List<BlogEntyTreeItem> GetTree()
   {
       NWDataContext db = new NWDataContext();
       var p = db.Posts.ToList();

       var list = p.GroupBy(g => g.DateCreated.Year).Select(g => new BlogEntyTreeItem
       {
           Text = g.Key.ToString(),
           Children = g.GroupBy(g1 => g1.DateCreated.ToString("MMMM")).Select(g1 => new BlogEntyTreeItem
           {
               Text = g1.Key,
               Children = g1.Select(i => new BlogEntyTreeItem { Text = i.Name }).ToList()
           }).ToList()
       }).ToList();

       return list;        
   }
}