Checkbox that pulls content from a custom post type

Question

I have a custom post type knits (here will be registered all knitwear products) and have another custom post type products (where will be listed all products).

What I would like to do: in this custom post type of products have a custom checkbox field listing up all items from custom post type knits. How can I do that?

NOTE: I don’t want to use categories.

Answer 1

The way to do this is to add a custom Meta Box with add_meta_box() and within the box list your CPT posts using get_posts(). And the information will be recorded as a Custom Field using update_post_meta(). Note that if Custom Field starts with a underscore (_NomeDoField), he is invisible in the standard custom Fields box.

In this example, the Meta Box is added to the regular Posts and the listing is of CPT Portfolio entries. Adjust as needed in the indicated locations.

<?php
/**
 * Plugin Name: (SOPT) Listagem de CPT em Meta Box
 * Plugin URI: /a/16674/201
 * Author: brasofilo
 */

add_action( 'add_meta_boxes', 'add_box_sopt_16606' );
add_action( 'save_post', 'save_postdata_sopt_16606', 10, 2 );

function add_box_sopt_16606() 
{
    add_meta_box(
        'sectionid_sopt_16606',
        __( 'CPT Associado' ), 
        'cpt_box_sopt_16606',
        'post', # <--- Ajuste CPT
        'side'
    );
}

function cpt_box_sopt_16606() 
{
    global $post;

    // Puxar todos as entradas do CPT
    $args = array(
        'numberposts' => -1,
        'post_type'   => 'portfolio', # <--- Ajuste CPT
        'post_status' => 'publish,future'
    );
    $get_posts = get_posts( $args );

    // Valor gravado ou default
    $saved = get_post_meta( $post->ID, '_cpt_associado', true );
    if( !$saved )
        $saved = array();

    // Segurança
    wp_nonce_field( plugin_basename( __FILE__ ), 'noncename_sopt_16606' );

    if( $get_posts )
    {
        foreach ( $get_posts as $cpt_post ) 
        {
            printf(
                '<input type="checkbox" name="_cpt_associado[%1$s]" value="%1$s" id="_cpt_associado[%1$s]" %3$s />'.
                '<label for="_cpt_associado[%1$s]"> %2$s ' .
                '</label><br>',
                esc_attr( $cpt_post->ID ),
                esc_html( $cpt_post->post_title ),
                checked( in_array( $cpt_post->ID, $saved ), true, false )
            );
        }
    }
    else
        echo '<strong>O CPT não tem entradas</strong>';
}


function save_postdata_sopt_16606( $post_id, $post_object ) 
{
    // Não é nosso tipo, sair fora
    if ( 'post' !== $post_object->post_type ) # <--- Ajuste CPT
        return;

    // Fazendo auto save?
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
        return;

    // Segurança
    if ( 
        !isset( $_POST['noncename_sopt_16606'] ) 
        || !wp_verify_nonce( $_POST['noncename_sopt_16606'], plugin_basename( __FILE__ ) ) 
        )
        return;

    // Gravar ou deletar
    if ( isset( $_POST['_cpt_associado'] )  )
        update_post_meta( $post_id, '_cpt_associado', $_POST['_cpt_associado'] );
    else 
        delete_post_meta( $post_id, '_cpt_associado' );
}

Upshot

_{Code adapted from List of Posts in a Custom Field}