Create a reddit-like infinite levels comment reply system [duplicate] - php

Question

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Generating Depth based tree from Hierarchical Data in MySQL (no CTEs)
(4 answers)
Closed 7 years ago.
Imagine an comment system with an infinite reply structure. Like this:
Comment
Reply to comment
Reply to reply to comment
Reply to reply to reply to comment
etc....
Reply to comment
Comment
Reply to comment
Like reddit has:
I am trying to think of a structure inside PHP combined with mysql to do this. I thought of something like this:
$query = mysqli_query($link, "SELECT * FROM comments");
while($comment_array = mysqli_fetch_assoc($query)){
echo $comment_array['text'];
$query_reply = mysqli_query($link, "SELECT * FROM comments WHERE reply_id='$comment_array[id]'");
while($reply_array = mysqli_fetch_assoc($query_reply)){
echo $reply_array['text'];
$query_reply2 = mysqli_query($link, "SELECT * FROM comments WHERE reply_id='$reply_array[id]'");
while($reply_array2 = mysqli_fetch_assoc($query_reply2)){
echo $reply_array['text'];
...... etc.
}
}
}
But as you can see there is a problem in this structure. This structure is not infinite and the same bit of code must be repeated a lot of times.
Is there a way to do this more efficiently? Putting a loop somewhere? Making a function, like searchRepliesofComment();?

Answer 1

There are a few ways to handle this without unbound recursion. A recursive function calling the DB repeatedly is not a good idea, even for reads. A few calls in a test environment may only take a few milliseconds each but even a little traffic can bring the DB to its knees when each request is firing off tons of DB hits.
First option you could have a post_id,parent_reply_id on all replies. The post_id would allow to select all replies that apply to a post but would have no hierarchy. Then you would build the reply tree in memory using parent_reply_id. Replies with a null parent_reply_id would represent top level replies. You could build this map easily with "SELECT * FROM replies WHERE post_id = ? ORDER BY parent_reply_id ASC". Ordering this way will make building the tree really straight forward.
You can also use the nested set model. Here is a great example. It does incur the cost of having to do a bulk (but relatively) light write operation on many replies when a new reply is added but the benefit is that you can build the tree with a single mysql query.

Answer 2

You will want to build what's known as a recursive function
function getComments($link, $parent = null)
{
$sql = "SELECT * FROM comments";
$sql .= $parent ? " WHERE reply_id=".(int)$parent : null;
$query = mysqli_query($link, $sql);
$results = array();
while ($result = mysqli_fetch_assoc($query)) {
if ($children = getComments($result['id'])) {
$result['children'] = $children;
}
$results[] = $result;
}
return $results;
}
function renderComments(array $comments)
{
$output = '';
foreach ($comments as $comment) {
$output .= $comment['text'];
if (isset($comment['children'])) {
$output .= renderComments($comment['children']);
}
}
return $output;
}
Now you can render your comments like:
renderComments(getComments($link));