WP CLI migration command from publishpress authors

This is the command previously used:

   /**
     * Misc commands to migrate data from PPA to authorship
     *
     * @subcommand migrate-authorship-data
     */
    public function migrate_to_authorship() {
        global $wpdb;

        // Migrate PPA terms to have the name and slug as the user id attached to each, to follow authorship conventions
        WP_CLI::line( 'Migrating PPA terms to Authorship terms: Updating name/slug to User ID.' );
        $wpdb->query( 'update  wp_terms as t1,
            ( select t.term_id, tm.meta_value from wp_terms t join wp_term_taxonomy tt on t.term_id = tt.term_id join wp_termmeta tm on t.term_id = tm.term_id and tm.meta_key = "user_id" where tt.taxonomy = "author" ) as t2
            set t1.name = t2.meta_value, t1.slug = t2.meta_value
            where t1.term_id = t2.term_id' );
        WP_CLI::line( 'Done.' );

        // Migrate PPA terms to switch their taxonomy to 'authorship' instead of 'author'
        WP_CLI::line( 'Migrating PPA terms to Authorship terms: Updating taxonomy of terms.' );
        $wpdb->query( 'update wp_term_taxonomy set  taxonomy = "authorship" where taxonomy = "author"' );
        WP_CLI::line( 'Finished the script.' );

        // Remove all term meta associated with previously-PPA terms
        WP_CLI::line( 'Migrating PPA terms to Authorship terms: Removing all related meta entries.' );
        $wpdb->query( 'delete from wp_termmeta where term_id IN ( select term_id from (
                select t.term_id from wp_terms t join wp_term_taxonomy tt on t.term_id = tt.term_id join wp_termmeta tm on t.term_id = tm.term_id and tm.meta_key = "user_id" where tt.taxonomy = "authorship"
            ) as t1 )' );
        WP_CLI::line( 'Done.' );

        // Migrate existing posts to set authorship relations for existing users, moving from using the post.author altogether
        WP_CLI::line( 'Migrating posts without authorship to include at least the main author.' );
        $posts_per_page = 100; // posts per page will be used for ticks
        $query_args = [
            'ep_integrate' => false,
            'post_type' => 'post',
            'post_status' => 'publish',
            'posts_per_page' => $posts_per_page,
            'paged' => 1,
            'tax_query' => [
                [
                    'taxonomy' => 'authorship',
                    'operator' => 'NOT EXISTS',
                ],
            ],
        ];
        $query = new WP_Query( $query_args );

        // New progress bar – provide number of all posts we'll be dealing with as well as a size of a batch processed before the first/next tick will happen
        $progress = \WP_CLI\Utils\make_progress_bar(
            sprintf(
                'Starting the command. Found %d posts',
                $query->found_posts
            ),
            $query->found_posts,
            $posts_per_page
        );

        $progress->display();

        do {
            $offset = $posts_per_page * $query_args['paged'];
            WP_CLI::line( sprintf( 'Processing %d posts at offset of %d of %d total found posts', count( $query->posts ), $offset, $query->found_posts ) );

            foreach ( $query->posts as $post ) {
                wp_set_object_terms( $post->ID, (string) $post->post_author, 'authorship' );
            }

            $progress->tick( $posts_per_page );

            // Free up memory.
            $this->stop_the_insanity();

            $query_args['paged']++;
            sleep( 5 );
            $query = new WP_Query( $query_args );
        } while ( $query->have_posts() );

        $progress->finish();

        WP_CLI::line( 'Finished the script.' );
    }

It has some issues:

the migrated authors are IDs not author names
no users are created for guest authors
only posts that do not have authorship data are processed
the command is destructive and will mangle or erase publishpress author data, requiring restoration of a backup, or data loss if no backup was made
it relies on SQL queries to perform many of its migration steps which can be opaque and unclear at first, perhaps to prioritise speed

My suggestion:

process all posts that have publishpress authors, rather than posts that do not have authorship data
do this via a paginated WP_Query
create missing users and terms as the process continues
present a report at the end of how many users were created, posts that were migrated
a second command for cleaning up the PPA data once migration is confirmed would be useful but outside the scope of this issue

humanmade / authorship

WP CLI migration command from publishpress authors #100