I have developed a plugin for myself and was using it for a little and after decided to go public. But the plugin got declined after submission and code review with the reason ##Calling core loading files directly. I fixed already all issues they mentioned, but only one bothering me now. I have require_once( ABSPATH.'/wp-admin/includes/upgrade.php' ); in few places to use dbDelta(), but if I will remove require_once declaration I won't be able to use dbDelta(). Do you think it will be an issue with second code review? Any developer who already did and released their plugins?
Short and simple:
It shouldn't be an issue.
Longer Answer: dbDelta() is a function that's kind of a special case, because the "core file" (upgrade.php) that defines it isn't always going to be loaded when it's needed by your plugin.
If it's a simple query, you could probably just use a prepared statement with $wpdb. However if dbDelta() is indeed better for your needs (and it sounds like it is), it's absolutely okay to use require_once with upgrade.php, despite it technically being a core file.
Take a look at the official Creating Tables with Plugins codex page that literally tells you to go ahead and use it in this manner:
require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
dbDelta( $sql );
With the reasoning:
[...] we'll use the dbDelta function in wp-admin/includes/upgrade.php (we'll have to load this file, as it is not loaded by default) [...]
So, require_once away my friend!
Alrighty, great news!
Plugin got approved and it's already public :)
Thank you, everyone, for the recommendations!
You can release your plugin php code with require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );