Sortable custom admin columns?

Thank you for the reply. But I can't make head or tail in the approach. Here is my code. How should I modify this to let me sort the "name" column?

<?php
class MM_Medlem_Custom_Admin_Columns extends MB_Admin_Columns_Post {
	/**
	 * MM_Medlem_Custom_Admin_Columns constructor.
	 */
	function __construct() {
		parent::__construct( 'mm_medlem', array() );
		add_action( 'pre_get_posts', array( $this, 'filter' ) );
	}

	public function columns( $columns ) {
		$columns = parent::columns( $columns );

		/**
		 * Add more column in a specific position
		 *
		 * @param string $position New column position. Empty to not specify the position. Could be 'before', 'after' or 'replace'
		 * @param string $target The target column. Used with combination with $position
		 */
		$position = 'after';
		$target   = 'mm_ddi-nr';
		$this->add( $columns, 'navn', 'Navn', $position, $target );

		return $columns;
	}

	public function show( $column, $post_id ) {
		switch ( $column ) {
			case 'navn':
				$navn = '';

				$fnavn = get_post_meta( $post_id, 'mm_fornavn', true );
				$mnavn = get_post_meta( $post_id, 'mm_mellemnavn', true );
				$enavn = get_post_meta( $post_id, 'mm_efternavn', true );

				$navn .= $fnavn;
				if ( $mnavn ) {
					$navn .= ' ' . $mnavn;
				}
				$navn .= ' ' . $enavn;

				echo $navn;
				break;
		}
	}

	function filter( $query ) {
		$query->set( 'orderby', 'mm_fornavn' );
		$query->set( 'order', 'ASC' );
	}
}

Hi again!

I think we misunderstood each other. What I want is that the column "Navn" is clickable and can be sorted ASC or DESC according to the user interaction, and not let it's order being defined in backend.

I solved it myself by digging in your code. The best solution is to apply a filter in the sortable_columnsfunction in base.php:

public function sortable_columns( $columns ) {
	foreach ( $this->fields as $field ) {
		if ( is_array( $field['admin_columns'] ) && ! empty( $field['admin_columns']['sort'] ) ) {
			$columns[ $field['id'] ] = $field['id'];
		}
	}
	$columns = apply_filters( 'add_sortable_columns', $columns );
	return $columns;
}

and tell my custom.php to add the filter:

function __construct() {
	parent::__construct( 'mm_medlem', array() );
	// Other code
	add_filter( 'add_sortable_columns', array( $this, 'add_sortable_columns' ) );
}

and

public function add_sortable_columns( $columns ) {
		$columns['navn'] = 'navn';
		return $columns;
}