speed up text based many-to-many updating - php

Question

TL;DR;
Is there a faster way of updating m:m records looping one by one, when ids are not known.
This is about using tags.
On the front-end, I've got forms that use text based input to list and update tags.
When the form is submitted a user record needs to have the tags updated by deleting and creating records in the m:m table
There are multiple tag types, so there are multiple tag input fields, but all are stored in same table.
because the current system is using the tags as string, once the form passes the input data back to the server, it is a CSV string.
The plan (plan because I'm updating from storing as a string) is to
join all tags $tags = implode(',', array_merge($tagset1, $tagset2));
get individual tags $update_tags = explode(',',$tags);
delete all tags for user in m:m user_tag table
loop through $update_tags as $tag_name
find tag_id from $tag_name
add record to m:m user_tag table for user_id and tag_id combination
in the past I've seen issues where the deleting happens, but some un-handled error causes updates not to go through. I suspect this can be mitigated by implementing rollbacks.
While the tag count for updating is fairly low on both sides (user will have less than 15, and there are less than 1000 tags total) I'm concerned about the looping aspect of this, as past experience has been a great indicator that sql within a loop is a great thing to avoid.

Answer 1

this is what I ended up doing ...
it's a bit different from what I intended.
I explode the string and compare to the view result (as array) and do two array_diff operations, one to find deletes and one to find insert.
there is one query for each type of operation, lookup user tags, insert user tags, delete user tags.
The only duplicate is the query that looks up tag ids based on the tag name, in which case it's not looping anyway.
public function update_tags($user_id, $tags)
{
// convert tags string to array
$update_tags = explode(',',$tags);
// find user's tags
$user_tag_view_reslt = $user_tag_view->find(array("user_id = ?", $user_id));
// reduce to array with tag names only
$user_tag_view_reslt = array_column( $user_tag_view_reslt, 'tag_name');
$remove_tags = array_diff($user_tag_view_reslt, $update_tags);
$insert_tags = array_diff($update_tags, $user_tag_view_reslt);
$this->remove_user_tags($user_id, $remove_tags);
$this->insert_user_tags($user_id, $insert_tags);
}
public function remove_user_tags($user_id, $tags_array)
{
$tags_string = "'".implode("','",$tags_array)."'";
$tag_model = new \Model\Tag();
$rows = $db->exec("SELECT tag_id FROM tag WHERE tag_name in($tags_string) ");
$delete_tag_ids = implode(',',array_column($rows,'tag_id'));
// check if there are any rows to insert before doing so
if($delete_tag_ids != ''){
$exec = $db->exec("DELETE FROM `user_tag` WHERE user_id = $user_id AND tag_id in ($delete_tag_ids);");
}
}
public function insert_user_tags($user_id, $tags_array)
{
$tags_string = "'".implode("','",$tags_array)."'";
$tag_model = new \Model\Tag();
$rows = $db->exec("SELECT tag_id FROM tag WHERE tag_name in($tags_string) ");
$insert_tag_ids = implode(',',array_column($rows,'tag_id'));
$sql_vals = array();
foreach($rows as $row){
array_push($sql_vals,"($user_id, ".$row['tag_id']." )");
}
$sql_vals_str = implode($sql_vals,",");
// check if there are any rows to insert before doing so
if($insert_tag_ids != ''){
$exec = $db->exec("INSERT INTO `user_tag` (user_id, tag_id) VALUES $sql_vals_str;");
}
}
requires PHP 5.5 or grater (due to array_column), it also won't work without adjusting and accommodating for the models whose code is not shown