Capability to edit own post, but not create new one

Question

i can't find anywhere how can i forbid to create new item, but allow editing of posts which was assigned to user.

Lets say in my system there is admin who creates some posts, then assigns some specific user as author of this post, but i cant make that user could only edit, it ether can edit and create new, or nothing.

PS. i dont want to use ANY plugin for this. Because this is my plugin.

Answer 1

I have the same requirement. I believe it's a bug. I have today raised a WordPress TRAC reporting my findings.

https://core.trac.wordpress.org/ticket/29714

Have a look and see if you've got the same problem.

Workaround

I have also developed a workaround that may allow you to make progress in the mean time.

Create a filter function for "user_has_cap". In that filter function, when the capability being checked for is that which you have defined ( e.g. "edit_your_cpts"), add the following logic.

global $pagenow;
if ( $pagenow == "edit.php" && isset( $_REQUEST['post_type'] ) ) {
        $pagenow .= '?post_type=' . $_REQUEST['post_type' ];
}

This will then allow the failing function ( user_can_access_admin_page() ) to return true.

Update 11th October. The first part of the workaround had some unwanted side effects. To overcome these you need some more code.

add_filter( "add_menu_classes", "oik_sites_add_menu_classes" );
/**
 * Implement "add_menu_classes" filter for oik-sites
 *
 * Part 2 of the Workaround for TRAC #29714  
 */
function oik_sites_add_menu_classes( $menu ) {
  global $pagenow;
  if ( false !== strpos( $pagenow, "edit.php" ) ) {
    $pagenow = "edit.php";
  }
  return( $menu );
}