How to add custom Fields in search for Wordpress backend?

Question

On a custom type page, in administration, it lists the project title and the date, and so the search works only in these two fields.

How to make that at the time of the search it looks also in the custom Fields? What files do I need to modify? What function should I use?

Answer 1

The solution lies in this answer from Wordpress Developers:
How to filter post Listing (in WP Dashboard posts Listing) using a custom field (search Functionality)?

First, you need to add the dropdown and text input by adjusting the Post Type in comparison to $typenow:

/**
 * Adicionar filtro de Custom Fields na tela /wp-admin/edit.php
 */
add_action( 'restrict_manage_posts', 'ba_admin_posts_filter_restrict_manage_posts' );

function ba_admin_posts_filter_restrict_manage_posts()
{
    global $wpdb, $typenow;
    if( $typenow !== 'post' ) // AJUSTE SEU POST TYPE
        return;

    $sql = 'SELECT DISTINCT meta_key FROM '.$wpdb->postmeta.' ORDER BY 1';
    $fields = $wpdb->get_results( $sql, ARRAY_N );
    ?>
    <select name="ADMIN_FILTER_FIELD_NAME">
        <option value=""><?php _e('Filter By Custom Fields', 'baapf'); ?></option>
        <?php
        $current = isset( $_GET['ADMIN_FILTER_FIELD_NAME'] ) ? $_GET['ADMIN_FILTER_FIELD_NAME'] : '';
        $current_v = isset( $_GET['ADMIN_FILTER_FIELD_VALUE'] ) ? $_GET['ADMIN_FILTER_FIELD_VALUE'] : '';
        foreach ( $fields as $field ) 
        {
            # Hidden fields
            if ( substr( $field[0], 0, 1 ) != "_" )
            {
                printf(
                    '<option value="%s"%s>%s</option>',
                    $field[0],
                    $field[0] == $current ? ' selected="selected"' : '',
                    $field[0]
                );
            }
        }
    ?>
    </select> 
    <?php _e('Value:', 'baapf'); ?>
    <input type="TEXT" name="ADMIN_FILTER_FIELD_VALUE" value="<?php echo $current_v; ?>" />
    <?php
}

Then add the filter when the search happens inside the page /wp-admin/edit.php and the $_GET correct is set:

/**
 * Fazer a filtragem conforme o $_GET
 */
add_filter( 'parse_query', 'ba_admin_posts_filter' );

function ba_admin_posts_filter( $query )
{
    global $pagenow;
    $get_url = isset($_GET['ADMIN_FILTER_FIELD_NAME']) && $_GET['ADMIN_FILTER_FIELD_NAME'] != '';
    if ( is_admin() && $pagenow=='edit.php' && $get_url ) 
    {
        $query->query_vars['meta_key'] = $_GET['ADMIN_FILTER_FIELD_NAME'];
        if (isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '')
            $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE'];
    }
}

Note that the search is literal by "Value" and that the Fields with arrays serialized are not considered.

---

I tested this other solution for filtering and seems to work with serialized Fields (deeper tests are needed):

add_filter( 'posts_clauses', 'wpse143477_parse_clauses', 10, 2 );

function wpse143477_parse_clauses($clauses, $query) {
    global $pagenow, $wpdb;
    $get_url = isset($_GET['ADMIN_FILTER_FIELD_NAME']) && $_GET['ADMIN_FILTER_FIELD_NAME'] != '';
    if ( is_admin() && $pagenow=='edit.php' && $get_url ) 
    {
        $keyword = $_GET['ADMIN_FILTER_FIELD_VALUE'];
        $keyword = '%' . like_escape( $keyword ) . '%';
        $post_ids = $wpdb->get_col( $wpdb->prepare( "
            SELECT DISTINCT post_id FROM {$wpdb->postmeta}
            WHERE meta_key = '%s' AND meta_value LIKE '%s'
        ", $_GET['ADMIN_FILTER_FIELD_NAME'], $keyword ) );
        if( !empty( $post_ids )) {
            $clauses['where'] .= " AND {$wpdb->posts}.ID IN (" . implode( ', ', $post_ids ) . ')';
        }
    }
    return $clauses;
}

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^{PS: This type of code can go functions.php, but it is more appropriate as a plugin.}