search based on custom field

Question

Add these filters to your functions.php file

/**
 * Join posts and postmeta tables
 *
 * @link https://developer.wordpress.org/reference/hooks/posts_join/
 */
add_filter( 'posts_join', function( string $sql, WP_Query $query ) {
    global $wpdb;
    if ( !is_admin() && $query->get( 'search_meta' ) ) {
        $sql .= ' LEFT JOIN ' . $wpdb->postmeta . ' ON ' . $wpdb->posts . '.ID = ' . $wpdb->postmeta . '.post_id ';
    }
    return $sql;
}, 10, 2 );

/**
 * Modify the search query with posts_where
 *
 * @link https://developer.wordpress.org/reference/hooks/posts_where/
 */
add_filter( 'posts_where', function( string $sql, WP_Query $query ) {
    global $wpdb;

    if ( !is_admin() && $query->get( 'search_meta' ) ) {

        $field_query = '';

        if ( is_string( $query->get( 'search_meta' ) ) ) {
            $field_name = $query->get( 'search_meta' );
            $field_query = "AND {$wpdb->postmeta}.meta_key = '{$field_name}'";
        }

        $sql = preg_replace( '/\(\s*' . $wpdb->posts . '.post_title\s+LIKE\s*(\'[^\']+\')\s*\)/', '(' . $wpdb->posts . '.post_title LIKE $1) OR (' . $wpdb->postmeta . '.meta_value LIKE $1' . $field_query . ')', $sql );
    }

    return $sql;
}, 10, 2 );

/**
 * Prevent duplicates
 *
 * @link https://developer.wordpress.org/reference/hooks/posts_distinct/
 */
add_filter( 'posts_distinct', function( string $sql, WP_Query $query ) {

    if ( !is_admin() && $query->get( 'search_meta' ) ) {
        return 'DISTINCT';
    }

    return $sql;
}, 10, 2 );

And then you just need to add the search_meta argument in your request. To search all meta values use 'search_meta' => true…

add_action( 'wp_ajax_data_fetch', 'data_fetch' );
add_action( 'wp_ajax_nopriv_data_fetch', 'data_fetch' );
function data_fetch() {

    $recipes = new WP_Query( [
        'post_type'     => 'project5',
        'orderby'       => 'rand',
        'post_status'   => 'publish',
        's'             => esc_attr( $_POST['keyword'] ),
        'search_meta'   => true
    ] );

}

Or to search a specific field value use 'search_meta' => 'MY_FIELD_NAME'…

add_action( 'wp_ajax_data_fetch', 'data_fetch' );
add_action( 'wp_ajax_nopriv_data_fetch', 'data_fetch' );
function data_fetch() {

    $recipes = new WP_Query( [
        'post_type'     => 'project5',
        'orderby'       => 'rand',
        'post_status'   => 'publish',
        's'             => esc_attr( $_POST['keyword'] ),
        'search_meta'   => 'keyword' // Name of custom meta_key
    ] );

}

This hasn’t been fully tested so please keep that in mind.

EDIT

A “less overkill” approach is to use the meta parameters in WP_Query (docs here). For example…

add_action( 'wp_ajax_data_fetch', 'data_fetch' );
add_action( 'wp_ajax_nopriv_data_fetch', 'data_fetch' );
function data_fetch() {

    $input = esc_attr( $_POST[ 'keyword' ] );

    $recipes = new WP_Query( [
        'post_type'     => 'project5',
        'orderby'       => 'rand',
        'post_status'   => 'publish',
        's'             => $input,

        'meta_key'      => 'MY_CUSTOM_FIELD_DATABSE_KEY',
        'meta_value'    => $input,
        'meta_compare'  => 'LIKE'
    ] );

}

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