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<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.


add_action( 'cmb2_admin_init', 'pb_cmb2_service_featured' );
function pb_cmb2_service_featured() {

	$prefix = '_pb_service_featured_';

	$cmb_service_featured = new_cmb2_box( array(
		'id'            => 'test_metabox',
		'title'         => __( 'Make Service Featured', 'cmb2' ),
		'object_types'  => array( 'pb_cpt_services', ), // Post type
		'context'       => 'side',
		'priority'      => 'low',
		'show_names'    => true, // Show field names on the left
		// 'cmb_styles' => false, // false to disable the CMB stylesheet
		// 'closed'     => true, // Keep the metabox closed by default
		) );

		// Regular text field
		$cmb_service_featured->add_field( array(
			// 'name'       		 => __( 'Make Service Featured', 'cmb2' ),
			'desc'       		 => __( 'Set to yes to move this service to the top of the services page. Set only one service as featured at a time.', 'cmb2' ),
			'id'         		 => $prefix . 'select_featured',
			'type'             => 'select',
			'show_option_none' => false,
			'default'          => 'No',
			'options'          => array(
				'no'	=> __( 'No', 'cmb2' ),
				'yes' => __( 'Yes', 'cmb2' ),
			),
			'column' => array(
				'position' => 2,
				'name'     => 'Featured',
				),
		) );

}

<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.


add_action( 'pre_get_posts', 'pb_change_services_query' );
function pb_change_services_query( $query ) {

	if( $query->is_main_query() && !is_admin() && is_post_type_archive( 'pb_cpt_services' ) ) {

		$meta_query = array (
			array(
				'key'     => '_pb_service_featured_select_featured',
				'value'   => 'yes',
				'compare' => '=',
			)
	);

	$query->set( 'meta_query', $meta_query );
	$query->set( 'orderby', 'meta_value' );
	$query->set( 'meta_key', '_pb_service_featured_select_featured' );
	$query->set( 'order', 'DESC' );
	$query->set( 'posts_per_page', '1' );

	}
}