Question

I found the below threaded comments example online and the author says it works very well for him. However, I’m having problems ordering the results so the threaded comments are in the right place. This is what the example gives me:

Example author says: “I use a system which is simple and doesn’t rely on recursion. I
basically store the entire thread “path” as a row field. Want to get
the entire tree structure? Just do an ORDER BY on the path column and
use some PHP code like the following for formatting:”

The Example Data

ID | Comment                      | Path
---+------------------------------+----------
0  | Comment #1                   | 00
1  | Comment #1 reply             | 00_01
2  | Comment #1 reply reply       | 00_01_02
3  | Comment #2                   | 00
4  | Comment #3                   | 00
5  | Comment #3 reply             | 00_04

The Example SQL

SELECT * FROM comments ORDER BY path

The Example PHP

while ($result = mysql_fetch_assoc($query)) {
    $nesting_depth = count(explode("_", $result['path']));
    $branch = str_repeat("--", $nesting_depth);
    echo $branch {$result['comment']}";
}

The Example Result

Comment #1
-- Comment #1 reply
---- Comment #1 reply reply
Comment #2
Comment #3
-- Comment #3 reply

I added the exact data to my MySQL database and tested it and worked as expected. However, if I change the order of the data in the table, it does NOT work. I’ll explain:

New REALISTIC Test Data

ID | Comment                      | Path
---+------------------------------+----------
0  | Comment #1                   | 00
1  | Comment #2                   | 00
2  | Comment #3                   | 00
3  | Comment #3 reply             | 00_04
4  | Comment #1 reply             | 00_01
5  | Comment #1 reply reply       | 00_01_02

Take a look at row[4] and row[5], these reply comments were the last comments added and drastically changes the order of results:

New Test Results

Comment #1
Comment #2
Comment #3
-- Comment #1 reply
---- Comment #1 reply reply
-- Comment #3 reply

This is a big problem! Is this guy talking absolute rubbish or have I done something wrong? Unless the data is in the exact same order you want to display, it will never work. Is there something simple I can do to fix the order?

Answer 1

Every “main” comment must have a path that contains a unique ID number. In your case every “main” comment has an ID of 00. If you have three of those, then there’s no way to get a reply in between.

ID | Comment                      | Path 
---+------------------------------+---------- 
0  | Comment #1                   | 00 
1  | Comment #2                   | 00 
2  | Comment #3                   | 00 
4  | Comment #1 reply             | 00_01 <-- Last item

The last item will alway be the last item (alphabetically). If you distinguish every “main” comment with a unique ID, then the problem is solved.

ID | Comment                      | Path 
---+------------------------------+---------- 
0  | Comment #1                   | 01
1  | Comment #2                   | 02 
2  | Comment #3                   | 03 
4  | Comment #4 reply to 1 (1)    | 01_01 <- first key is parent_id, second is sequence
5  | Comment #5 reply to 1 (2)    | 01_02 
6  | Comment #6 reply to 4 (1)    | 04_01 
7  | Comment #7                   | 04 
8  | Comment #8 reply reply to 5  | 01_02_01 

So when you’re going to reply to a comment all you need to do is reference their entire path, and add an indexing key at the end.

In the above table:

01_01 = parent id: 01 -> sequence: 01
04_01 = parent id: 04 (so a reply to id 4) -> sequence: 01
01_02_01 = parent_id: 01_02 (references the path of ID 5) -> sequence: 01

etc.

Then again, this is a weird construction. In my opinion an id/parent_id relation is better for these kind of things.