Implementing recursive property loading in EF Core

Question

I'm on .NET Core 1.1.0, EF Core 1.1.0, VS 2015.

I'm writing a system for posts/comments, and I need a function to load a comment and all of its children and their associated properties. Here's a simplified version of my classes:

public class Comment
{
    public long Id { get; set; }

    public string Content { get; set; }

    public User User { get; set; }

    public ICollection<Comment> Replies { get; set; }
}

public class User
{
    public long Id { get; set; }

    public string Name { get; set; }

    public Avatar Avatar { get; set; }
}

public class Avatar
{
    public string Url { get; set; }
}

Any given comment can have any number of replies:

-PARENT
    -CHILD 1
    -CHILD 2
        -CHILD 3
    -CHILD 4
    -CHILD 5
        -CHILD 6
            -CHILD 7

So, given the ID of the parent comment, I need to load the entire tree, including users and their respective avatars. (I have controls elsewhere to make sure these trees don't become unwieldy, I'm not concerned at this point about potentially grabbing way too much data.)

The Loading Related Data page in the EF Core docs is very helpful, but I'm not sure how to best handle this. I've experimented putting some things together but I just can't conceptualize how to fit it all together. To note again: I'm using EF Core 1.1.0, so I do have access to the functions in the "Explicit loading" section.

How can I load the entire tree of comments given a parent comment's ID?

Solution 1

I do not have a database so I just did it in memory but if you follow my comments, it will work for you. Notice the objects I have in memory, only comment with id 2 has replies.

LoadComment method is where everything happens. The rest is just setup code I needed.

class Program
{
    static void Main(string[] args)
    {
        var result = LoadComment(1, null);
        Console.ReadKey();

    }



public static Comment LoadComment(long id, Comment com) 
{
   Comment res = new Comment();
   if( com == null ) 
   {
      // You would call your context here and write db.Single(x => x.Id == id).Include(x => x.User.Avatar);
      var first = db.Single( x => x.Id == id );

      res = new Comment { Id = first.Id, Replies = first.Replies.ToList(), User = first.User };
      foreach( var item in first.Replies ) 
      {
         LoadComment( item.Id, item );
      }
   }
   else 
   {
      // You would call your context here and write db.Single(x => x.Id == id).Include(x => x.User.Avatar);
      var child = db.SingleOrDefault( x => x.Id == id );
      if( child == null ) 
      {
         return null;
      }
      com.Replies = new List<Comment>();
      com.Replies.Add( new Comment { Id = child.Id, Replies = child.Replies.ToList(), User = child.User } );
      foreach( var item in child.Replies ) 
      {
         LoadComment( item.Id, com );
      }
   }


   return res;
}

    private static Comment cm1 = new Comment
    {
        Id = 1,
        User = new User { Id = 1, Avatar = new Avatar { Url = "1" } },
        Replies = new List<Comment> {
        new Comment { Id = 2 },
        new Comment { Id = 3 },
        new Comment { Id = 4 },
        new Comment { Id = 5 } },
        Content = "ContentForCommentId1"
    };

    private static Comment cm2 = new Comment
    {
        Id = 2,
        User = new User { Id = 2, Avatar = new Avatar { Url = "2" } },
        Replies = new List<Comment> {
        new Comment { Id = 22 },
        new Comment { Id = 33 },
        new Comment { Id = 44 },
        new Comment { Id = 55 } },
        Content = "ContentForCommentId2"
    };
    private static List<Comment> db = new List<Comment> { cm1, cm2 };

}

Solution 2

This is how I solved it. Fairly similar to Yoshi's but could be helpful to someone.

private async Task<Comment> GetComment(Guid id, CancellationToken cancellationToken)
{
    var wm = await _context.Comments
                           .Include(x => x.Replies)
                           .SingleOrDefaultAsync(x => x.Id == id, cancellationToken);

    for (var i = 0; i < wm.Replies.Count; i++)
    {
        if (!wm.Replies[i].IsDeleted)
            wm.Replies[i] = await GetComment(wm.Replies[i].Id, cancellationToken);
    }

    return wm;
}

Comments

[ESG]

The round-trip to the database for each children might be quite significant. Might be better to do a recursive CTE using SQL and have EF map the results.

[vaindil]

This looks very promising! I'm getting this error though in the last foreach: Collection was modified; enumeration operation may not execute. It happens after loading the first comment's children; the second item in the foreach triggers the exception.

[CodingYoshi]

Sorry there was a small bug in the if and else block. Please retry with the new code in LoadComment method. If it does not work, then I would have to create a database and try to test it that way.

[vaindil]

I ended up using this method, just tweaked a little bit. The updated code you provided works perfectly, I just changed a couple things for my codebase. Thank you so much!